A 5-0 aggregate win over Icelandic minnows KR Reykjavik in the first tie showed promising signs of progression into the group stages of the UEFA Champions League, that was until the Bhoys were drawn against Legia Warsaw.
Celtic headed to Poland for the first leg against the Ekstaklasa champions, but were swept aside by Henning Berg's men, losing 4-1. It could have gone much worse for the Hoops that night; Legia captain Ivica Vrdoljak missed not one but two penalties and Nigerian defender Efe Ambrose was sent off.
Celtic needed a miracle in the second leg at Murrayfield, but Deila's side didn't put up any kind of fight as they conceded two unanswered goals, going out of the competition on a 6-1 aggregate defeat.
That miracle did go to work, albeit after the Warsaw tie. UEFA investigated the Polish club when it emerged that they fielded an ineligible player in the second leg, when Bartosz Bereszynski was subbed on for the final two minutes.
UEFA found Legia guilty, meaning they had to forfeit the second leg, rendering the result as an automatic 3-0 victory for Celtic, which was enough to see them through on away goals - perhaps a tad harsh on Legia considering the defender did only play two minutes of football.
The Scottish champions were then cleared to play NK Maribor of Slovenia, with many Celtic fans hoping to see new faces on the pitch before the tie. The writing was on the wall following the Legia tie, new signings were absolutely imperative if Celtic were to have any hope of Champions League qualification, which would have been their third season in a row in Europe's premier competition.
However the only signings made all summer by chief executive Peter Lawwell have been loan deals for Aleksaner Tonev, yet to make an appearance in green and white, Jason Denayer and Jo Inge Berget. The only permanent signing Celtic have made this summer is goalkeeper Craig Gordon, who was a costless agent.
It wasn't as if Celtic didn't have money to spend; Fraser Forster was sold to Southampton for £10m only a few weeks ago, plus the Hoops accumulated £11m for winning the Scottish Premiership last season and for reaching the play-off round of Champions League qualifying.
The Hoops headed to Slovenia where they earned a 1-1 draw, which wasn't a terrible result to bring back to Celtic Park, if anything they were in the driving seat after the first leg.
Ronny Deila opted to rest 10 players in the league game against Inverness the weekend before the second leg against Maribor, as his side lost 1-0. But when the big night arrived, he made some odd decisions as to who started and who didn't.
Kris Commons, one of Celtic's most consistent players started on the bench, while Berget, who hasn't looked up to speed in the handful of appearances he's made, started on the left of midfield.
Anothony Stokes also started as a centre forward, despite being more of a number 10 as he thrives playing alongside a deep lying forward à la Gary Hooper. Although, Celtic have desperately lacked a poacher ever since Hooper's departure.
Deila didn't seem to execute the tactics properly in the return leg either. From the off the Hoops looked slow and sluggish on the ball and struggled against a compact Maribor defence.
As a result, the Hoops created very few chances and were slow getting back whenever the guests recovered possession. Ambrose in particular looked lost at right back, failing time after time to pick up runs from the flank.
Inevitably, Maribor took the lead on 75 minutes, when Tavares struck home after the Celtic defence failed to clear their lines. From that point, it was all route one football from the Hoops.
Virgil van Dijk was thrown up to the other end of the park as a target man and missed a great chance in stoppage time. The Dutchman chested down a long ball inside the six yard box but hit it straight at the goalkeeper, seemingly toe poking the ball.
The defender's future at Celtic now seems to be in doubt. Without Champions League football, Van Dijk has no raison d'être in the East End of Glasgow; a player of his quality who has been linked to various Premier League teams strives for bigger and better things than the Scottish Premiership.
This is why, for a club like Celtic, Champions League qualification is vital. Without Champions League nights at Celtic Park, it becomes twice as difficult to attract quality players to the club.
It also becomes difficult to attract support. With expectations on European football so high, it is often seen as a forfeit among the Celtic support to qualify for the Europa League and so there is a plummet in attendances.
Many would argue that the current Celtic squad are only Europa League standard, which is a tad generous as a matter of fact. Though, the embarrasment Legia Warsaw - both the thrashing and the ensuing debacle - as well as the defeat to Maribor could easily have been avoided had Lawwell and co. decided to put their hands in their pockets.
Around 200 supporters stayed behind at Celtic Park following the 1-0 defeat to Maribor to vent their frustration at a board which they believe is only interested in money and not on field success.
Indeed, it would be incredibly harsh to shift all of the blame on Ronny Deila. Yes his team selection was questionable. Yes his tactics were poorly executed, but the fact remains, the Norwegian didn't have the necessary materials to work with.
Even Gordon Strachan was humbled in his first shot at Europe with Celtic, losing 5-4 on aggregate to Slovakian side Artmedia Bratislava. But in his next two years, Strachan took the Hoops to the last 16, producing memorable results such as the 1-0 win over Manchester United at Parkhead.
Who knows, perhaps this summer's European ventures - or rather, lack of them - could be the making of Ronny Deila at Celtic.