Liverpool star Mohamed Salah and Manchester City midfielder Kevin De Bruyne's forgettable spells at Chelsea remain one of the mysteries of recent Premier League history and a not-trusting-youth-shaped stick to beat now-Tottenham Hotspur manager Jose Mourinho with. 

After a stunning loan spell at Werder Bremen saw him named the Bundesliga's Young Player of the Year for 2012/13, De Bruyne was integrated into Mourinho's first team squad for the following season, but he only made two Premier League starts for Chelsea and then left to join Wolfsburg in January. 18 months later, he'd sign for Manchester City and establish himself as arguably the best midfielder in the world. 

Salah's Stamford Bridge spell was similarly underwhelming. Despite being one of the deadliest goalscorers in the Premier League and this season leading the Golden Boot race after firing Jurgen Klopp's side to last term's title, he only managed 19 appearances for the Blues after signing from FC Basel. Instead, much of his time with the west London club was spent out on loan. 

Mourinho's lack of faith in developing players is often seen as the key reason why neither made much of an impact at Stamford Bridge, but former Chelsea star Oscar - who was above De Bruyne and Salah in the pecking order - paints a slightly different story. 

Speaking to Yellow And Green Football, he said: "De Bruyne even gave me an assist, if I'm not mistaken. There was a formation where Eden Hazard, De Bruyne and me played in midfield and it worked well. He only had a few chances, played well but then got injured. I guess not having that consistency at a club that had many options in the centre of the park meant he wasn't a good fit at the time. 

"With Salah, it was different. He was unstoppable during practice, but always shy and quiet off the field. I guess he did not have the confidence during games that we saw at every club he played for after Chelsea, and especially now at Liverpool."

So while some may criticise Mourinho for not seeing their true potential, Oscar offers an alternative perspective of De Bruyne and Salah not being physically and mentally ready respectively to be key players for Chelsea at that time. Let's not forget Chelsea would win the 2014/15 Premier League title under Mourinho, so there was plenty of experience and quality already in the squad. 

Nonetheless, it remains one of Chelsea's biggest recent mistakes in the transfer market as Liverpool and Manchester City continue to dominate English football.