With Liverpool starting the 2014/15 season using a back four, Mamadou Sakho struggled to find a way into Brendan Rodgers' side.

The French international only featured in three of The Reds' opening 16 Premier League matches and was reported to be very unhappy with his lack of game time. However, Liverpool's poor first half of the season saw Rodgers change to a back three.

In this new system, Sakho cemented his place in a trio that was integral to Liverpool's 2015 rejuvination. Playing alongside Martin Skrtel and Emre Can, Sakho helped to solidify Liverpool's leaky back line and start a push for a top-four finish.

A recent injury picked up away to Arsenal though has sidelined the defender and caused for more change at the back, leading to the end of the challenge for a Champions League place.

Firstly, it would be wrong to say that if Sakho had stayed fit, Liverpool would still be in the running for 4th place. Dejan Lovren has come into the side in Sakho's absence and performed much more like he did last season for Southampton than how he was doing in his first five months at Liverpool. It's more a case of Liverpool being forced to change from a system that was working tremendously well for them, because of this injury.

If Liverpool were to try and stick with the back three without Mamadou, they would be forced to play with three right footed centre backs, which can cause issues itself. Along with this, Sakho's style of play suited the back three a lot more than Lovren, who looks much more comfortable when playing in a four.

Sakho likes to push the defensive line out and defend aggresively, which you can do much easier in a system where he has two other centre backs there to cover should he make an error. Lovren on the other is more reserved in his style of defending. His strength in reading the game along with his weaknesses with regard to pace and mobility make him much more suited to playing in a system where a fullback can protect him on the outside.

Due to this, the decision to change is understandable, but it has also been quite costly. As well as solidifying the defence, the 3-4-3 allowed Liverpool to be much more fluid in attack. The more recent 4-3-3 and 4-2-3-1 formations that have been installed since Sakho's injury have become a lot more rigid and one dimensional, preventing The Reds from getting the results needed to continue competing for the top-four. 

It is not all just systematic though. Mamadou Sakho has shown this year just how good a defender he can be, answering a lot of critics who doubted him after a nervey opening season in England. Liverpool have only lost four league games this campaign with Sakho in the side. His influence on the team was evident seeing as he was brought back into the fold at the start of Liverpool's 13 game unbeaten run that saw them quickly climb from their midtable slumber.

When Liverpool were on their strong run of form, Brendan Rodgers was quick to praise the Frenchman who had been rumoured to have fallen out with his manager earlier in the season. Rodgers said that despite still having improvements to make in his game, Sakho clearly does care and that on the pitch he was like a warrior.

It may have taken longet than he would have hoped, but Sakho clearly now has the faith of his manager, which is perhaps where some of Liverpool's other defenders are behind. His confidence during what was his most consistent run of games since joining the club was there for all to see. A young defender with great leadership qualities and also an appropriate amount of arrogance in his play, Sakho is definitely now a player that Liverpool need to have available.

It's important to remember that saying that Mamadou Sakho has been missed by Liverpool isn't saying that the players who have played in his absence have been poor. Martin Skrtel and Dejan Lovren have both performed very well in the last month, but what Sakho brings to the team has taken a lot out of The Reds' game.

Very much a 'Brendan Rodgers type of defender,' Sakho was pivotal to most of the positive changes going on at Liverpool around the turn of the year, and now that he is out of the side the side is struggling to find it's form again. There is no question that when Sakho is back to fitness that he will slot almost straight back into the side, and this is testemant to just how crucial he is to the defence and how much he has been missed.