Chelsea have spent £122m on left backs since Ashley Cole departed in 2014... so what happened to the costly stars who failed to replace him?

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Chelsea look set to smash their transfer record with the £61million purchase of Juventus left-back Alex Sandro, beating the £50m mark set by Fernando Torres' arrival in 2011.

With rapid pace and a penchant to attack, Sandro has naturally drawn comparisons with former Chelsea star Ashley Cole. Should he live up to that comparison, it will bring an end to a long and expensive search to find Cole's successor.

Indeed, the Blues have signed three left backs in the last three summer windows in an attempt to fill his boots. Sandro's arrival will take spending on left backs since 2014 up to an incredible £122m.

Sportsmail runs through the three stars who have tried and failed to live up to expectation...

Ashley Cole departed Chelsea in 2014 but the gap he left in the team still remains

Filipe Luis (signed from Atletico Madrid - £16m - 2014)

Luis had been inspirational for Atletico Madrid en route to their La Liga title win in 2013-14 and was considered among the best left backs in Europe by the time the season concluded.

Jose Mourinho made a move to sign him along with Diego Costa, but Luis struggled to settle into his new surroundings in west London.

Mourinho's patience quickly wore thin and Cesar Azpilicueta soon emerged as a genuine alternative for the left-sided position, limiting Luis to just nine Premier League starts.

He made the move back to Atletico Madrid one year later with Chelsea managing to recoup £11m of their original outlay. He remains an important player in Diego Simeone's team. 

Filipe Luis struggled to settle into life at Stamford Bridge and quickly returned to Spain

Baba Rahman (signed from Augsburg - £21.7m - 2015)

With a spectacular campaign for Augsburg under his belt, the 20-year-old Rahman was being courted by some of Europe's biggest clubs in the summer of 2015 and Chelsea's successful bid to sign him was considered quite the coup.

Once again, Mourinho never appeared to warm to his summer signing, choosing to continue with Azpilicueta before his sacking as manager in December. Rahman did, however, begin to break through under the management of Guus Hiddink.

He finished the campaign with 11 Premier League starts but Antonio Conte did not appear convinced, quickly shipping the Ghanaian out on loan to Schalke where he suffered a serious knee injury early on. 

He remains on Chelsea's books but is expected to leave this summer with Everton one of a number of clubs interested.

Baba Rahman started 11 Premier League games for Chelsea following his £22m move

Marcos Alonso (signed from Fiorentina -£23m - 2016)

Arguably the most surprising signing of the trio yet by far the most successful, Alonso was more-or-less unknown in English football prior to his deadline-day arrival at Stamford Bridge.

Quickly labelled a panic buy by most pundits, Alonso soon proved them wrong by embedding himself in the 3-4-3 formation. He went on to start 31 Premier League games, scoring six goals and providing a further three assists.

In fact, it is his success in the role that makes Chelsea's move for Sandro all the more surprising. The former Bolton and Sunderland player has every right to feel hard done by if the Brazilian pushes him onto the bench.

Marcos Alonso was a regular player for Antonio Conte in Chelsea's title-winning season

Alex Sandro (signing from Juventus - £61m - 2017)

Yet to officially sign but only a few dotted I's and crossed T's from becoming the latest left back to turn up at Stamford Bridge.

While the extroverted Dani Alves took most of the plaudits for his performances on Juventus' right, Sandro got on with his business in equally impressive fashion on the other side of the field.

Alongside Marcelo and David Alaba, Sandro is considered one of the best left-sided defenders in world football and will certainly represent an upgrade on Alonso.

Alex Sandro is set to join Chelsea in a club record £61m deal from Juventus