One of the biggest and most successful clubs of the Premier League era, Chelsea have, for the most part, used the transfer market to their advantage over the past three decades.Over the years, Chelsea have been able to bring some of the world’s best players to Stamford Bridge. But they’ve also made many mistakes when it comes to transfers.The Blues have over-spent on big money flops, and they’ve also let players, who would go onto become Premier League legends, out of their grasp.On that note, let's take a look at five of the biggest transfer mistakes in Chelsea’s history.#1 Fernando Torres – Liverpool to Chelsea (£50m) January 2011Fernando Torres failed to live up to his £50m price tag at Chelsea.When Chelsea decided to part with a British record fee of £50m to bring Fernando Torres from Liverpool to Stamford Bridge in January 2011, it sounded like a good idea.‘El Nino’ had practically been a goal machine for the Reds since arriving at Anfield in the summer of 2007. His first three seasons at Liverpool saw him score 56 goals in just 79 appearances.However, what Chelsea’s power-brokers didn’t realise was that after suffering a serious knee injury in the latter part of the 2009-10 season, Torres wasn’t quite the same player he once was."Fernando Torres, Liverpool's number nine" 🎶@Torres was unstoppable #OnThisDay in 2010 as @LFC beat Chelsea 2-0 pic.twitter.com/swEHFkNlN1— Premier League (@premierleague) November 7, 2019On his arrival at Stamford Bridge, not only had the Spaniard lost a yard of pace, but he also looked like he had the weight of the world on his shoulders. It took him 14 games to open his goal account for Chelsea, and that strike turned out to be his only one in his first campaign for the Blues.His record improved in 2011-12, as he scored 11 goals in all competitions, but he also went on a 24-match barren run in the same campaign.By the 2012-13 season, despite scoring 22 goals, Torres was largely regarded as one of the Premier League’s all-time most expensive flops. And in 2014, he departed Chelsea for Milan, having scored just 20 Premier League goals for the Blues in 110 appearances.#2 Mohamed Salah – Chelsea to Roma (£13.5m) August 2016Mohamed Salah went onto become a Premier League legend, but only after being sold by Chelsea.Since joining Liverpool in the summer of 2017, Mohamed Salah has become a Premier League legend.The Egyptian striker has scored a phenomenal 90 goals in 135 league appearances for the Reds, winning the Premier League Golden Boot twice.He has helped them to two Champions League finals – winning the 2019 edition – and in 2019-20, played a huge part in Liverpool capturing their first Premier League title. Incredibly though, none of this would’ve been possible had Chelsea fully recognised his talent a few years ago.Salah first came to prominence in the eyes of Premier League fans in the 2012-13 season, when he scored Europa League goals for Basel against both Tottenham Hotspur and Chelsea, helping them to the competition’s semi-finals.Liverpool were actually linked with a move for Salah at that stage, but instead, he moved to Chelsea in January 2014 for a fee of £11m. Quite why the Blues chose to sign the Egyptian remains a mystery, as they already had a host of wide forwards at the time, including Eden Hazard, Oscar and Willian.Unsurprisingly, Salah struggled to force his way into Chelsea’s starting XI and made just 11 appearances in all competitions during his first half-season at the club, most of them off the bench.The 2014-15 season didn’t see him fare any better, and after appearing in just three Premier League games, he was sent on loan to Serie A side Fiorentina.Salah scored six goals in 16 games for the Italian side, which was enough to impress AS Roma – who took the Egyptian on loan in 2015-16 and signed him permanently for just £13.5m.After scoring 29 goals in just 65 appearances for Roma, Liverpool made their move, and the rest is history.25 - Mohamed Salah has scored more goals than any other Premier League player in all competitions this season (25). King. pic.twitter.com/LgAlLrLkB9— OptaJoe (@OptaJoe) March 10, 2021Given that Salah is likely worth well over £100m at this point, it’s safe to say that Chelsea made a major error with their treatment of a player who would end up becoming a world-class performer.