However, a number of high-profile mistakes in his first two campaigns has meant the club were forced to delve back into the market for a new No.1, despite paying a world-record fee for a goalkeeper two years prior, in order to sustain their title-winning ambitions.
Rather than forcing a move away from SW6 this summer - like many predicted - Arrizabalaga has stayed at Chelsea.
Some would argue this is circumstance over choice due the current state of the transfer market. Yet, over £1 billion was spent by Premier League teams this summer including various loan dealings.
So if Kepa or Chelsea wanted him to move elsewhere, surely a suitor could have been found?
This indicates that the Spanish shot- stopper still believes his future can be in Chelsea blue and that the club are not prepared to let the 26 year-old leave just yet.
With Kepa struggling to justify his record-breaking fee, almost every pundit was hammering on that Chelsea need a new goalkeeper if they were serious about winning a Premier League title and having success in both Europe and domestic competitions in the coming campaigns- and rightly so.
Last season, Kepa saved only 54.5% of shots on target, the lowest figure for any regular top-flight goalkeeper over a full season since [Opta] records began in 2003-04.
A main criticism that Kepa has faced since making his debut in August 2018 was his perceived inability to save shots from long range.
This assumption is warranted mind you, as Kepa conceded 19 goals from outside the box in the Premier League since his debut - more than any other goalkeeper.
Often referred to as proof of his unreliability, Kepa's overall save percentage was 53.47%- placing him bottom of the Premier League among goalkeepers with at least 1,000 minutes and almost 10 per cent lower than the next worst Aston Villa loanee and former Liverpool ace Pepe Reina, who is now back applying his trade in Italy with Lazio.
Though, it must be made clear that Chelsea were shambolic defensively in 2019-20 and have continued to show thier defensive frailties- before Mendy arrived- against Liverpool and West Bromwich Albion this season.
Since the start of last campaign, Chelsea have conceded more away Premier League goals than any other side - ahead of now championship side Norwich City  who were relegated last season.
The last 12 months have not been good for Kepa by any means, and it is extremely improbable that he will live-up to the expectations set by his record-breaking price tag-in which he had no control over.
But with age on his side and his whole career ahead of him, their is evidence that Kepa is a top-flight goalkeeper- as the 2018/19 season would suggest.
The season before last.
The scepticism surrounding Kepa's ability appropriately originates from his performances under Frank Lampard.
However, those who claim Kepa has never been good since he arrived at Stamford Bridge are incorrect.
Despite Kepa’s issues seeming unfixable, the 2018-19 season provides recent evidence that he is not as bad a goalkeeper as the one seen over the last year.
Kepa wasn’t a liability to Chelsea then- though the numbers suggest he was a slight negative after conceding 3.35 goals more than expected- and proved his worth while helping Chelsea to their second Europa League trophy.
His shots‑on‑target saved percentage was 67.5%, roughly the same as the 68.2% he recorded over two seasons in La Liga at Athletic Bilbao, providing zero obvious examples of him conceding shots that he should be saving.
Despite Arrizabalaga’s first season being more then fine, the problem lies in the depilation of trust amongst the club’s fans, the manager (who was brought to the club by predecessor Maurizio Sarri) and even his defence because the errors he has made of late
So why the sudden decline?
Why the drop in form?
As his save percentage and goals conceded numbers show, Kepa has a significant flaw in his diving technique.
When preparing to make a save, he swings his arms behind his body before leaping to save the oncoming ball.
The time it takes for him to bring his arms back in front of his body takes a split-second longer then any other goalkeeper, equalling more shots hitting the back of his net.
Another criticism aimed at Kepa is that he does not command his area, particularly at set-pieces.
Compared to Mendy's 6ft 6in frame, Kepa's 6ft 1in, 194lbs build does not help his cause when it comes to claiming crosses with confidence.
Can Kepa make a comeback?
Kepa could easily sit on Chelsea's bench as their back-up option, see out his contract and collect his £150,000 a week wage.
As evidenced by this summer’s market, no other club will be willing to afford that sort of income.
But he’s reacted well to Lampard's past benchings and his backing of him to the media, despite his demise.
"We have to keep working, Kepa has to keep working and he has to have support around him, that's very clear." said Lampard.
"Individually players need support, particularly from myself, and I give that to all the players.
"And now we've got to try to give him confidence, because that's important.
"Nobody tries to make mistakes in football, that's the nature of the game."
Petr Cech, now Chelsea's technical and performance advisor, has insisted that the club are not ‘giving up’ on Kepa.
Cech explained: “Kepa struggled last season, that is unquestionable."
“But the club invested in him and gave him a long-term contract because they believe he has the qualities and attributes to be successful.
"Nobody is giving up on Kepa and we all hope he has a great future.”
Rob Green, who hung-up his boots (or should I say, gloves) at Chelsea last season aged 40, also believes that Kepa still has a future at the West-London club.
“But because of that price tag, you’re expecting that bit more. That’s the immediate assumption, he’s not the finished article, that was never the case with Kepa.
Mendy may be the starter by default at the moment, but it’s a long and busy season ahead and, presumably if things go in Kepa's favour, a long Chelsea career awaits.
Look at James Rodriguez at Everton this season.
A player who burst onto the world stage at a young age, got a big money move to one of Europe's elite, and was made an outcast.
Now under Carlo Ancellotti, he is ripping up the Premier League.
And the same story could be said for Kepa in a few seasons time.
Stranger things have happened!
Why Mendy can improve Kepa.
In hindsight, the signing of Mendy is good for all parties- Chelsea improve the position they were so desperate to address (besides left-back and maybe even centre-back), Mendy gets a move to one the biggest clubs in world football and Kepa gets a chance to step-out of the spotlight and improve.
Mendy's arrival as the new No.1 will still have on an effect on Kepa- he will provide competition in training and will make him have a point to prove.
So having a new face will subconsciously raise the bar in the goalkeeping department and could actually help Kepa, not only at club level but internationally with the Euros coming up next summer.
In the life of a footballer there are ups and downs," Kepa said.
"But in the end what is in my hands is to work."
"I feel good, strong, and confident. During your career sometimes you have to live moments like this
Playing for Spain
Kepa is currently away with Luis Enrique's Spain squad for games against Portugal, Switzerland and Ukraine over the next seven days.
Not only is this a good escape for the troubled goalkeeper, but good news for Lampard who will be keen for Kepa to get as many minutes as possible at the highest level while still being a Chelsea player
He made some key saves throughout Wednesday's warm-up game against Cristiano Ronaldo and Co, and was helped by the woodwork after Portugal saw shots crash off the bar.
He then went on to make a crucial stop to deny Barcelona new boy Francisco Trincao late in the game, after a slick counter-attack as Spain saw out the game to a goalless draw.
Kepa, who was a surprise starter ahead of David de Gea and Unai Simon, said: "I'm confident I can turn it around and do my best when I play. We all want to play. But I don't think about anything beyond day-to-day life and focusing on the national team's matches.
"I've felt confidence, at ease [with the national side].
“I think it was a good game and I'm happy with the confidence of the coach.
"When I'm needed to help on the pitch, I try to do the best possible.
"It was a game of great intensity.
His national coach Luis Enrique has encouraged Kepa to rise to the challenge and prove his talent and ability for the national team since being usurped at club level.
"Kepa is not in a nice situation, but it is not the end of the world," Enrique told a news conference on Tuesday.
"It's the ideal moment for him to rise again, for us to see what Kepa is made"
Is Kepa's future at Chelsea?
Kepa has made mistakes, as many young goalkeepers do.
His save rate hasn't been good, he's let in too many shots from range and his lack of commanded as been a problem.
The criticism from fans and pundits has not helped either, nor was the talk of his beneficiaries before Mendy eventually signed.
Relentless negativity has made it almost impossible for him to escape the downward spiral.
Not only does he have to mend his own form and restore confidence in his own ability, the summit of all that scrutiny he’ll have try and convince Chelsea supporters- who pleaded for a new No.1- that he's still a decent goalkeeper .
Yes, Mendy is that new man between the sticks and is all the rage at the moment. He's the savour from last seasons defensive horror show.
But until an incident arises he could be cast a different role- much like Kepa was.