When it comes to selling players, it is usually the aim to try and get rid of them when they are on a downward spiral.
Sir Alex Ferguson used to be brilliant at it, as he'd constantly refresh his successful Manchester United teams with new blood even after they'd won trophies, while it could also be said that Arsene Wenger sold the likes of Thierry Henry and Patrick Vieira just as they were past their best.
There is something of an art to it, but it is an art that Chelsea, despite their success, have never quite been able to master.
And with Manchester United's shock move for the former Blues forward Romelu Lukaku - who had seemed all set for a return to his old club this summer - they seem set to regret letting another one get away.
£10m signing Lukaku played just 15 games for Chelsea before they discarded him in 2012.
Loan spells at West Brom and Everton never convinced the Londoners that they wanted to keep him, but after completing his permanent move to the latter he's become a Premier League star.
And as the following list shows, he's a member of a group of players that might just have been written off too soon by their former employers, often with expensive results.
Kevin De Bruyne
The 2012 purchase from Genk was immediately tipped for big things at Stamford Bridge, but other than a brief spell under Jose Mourinho at the beginning of the 2013/14 season he was never given a consistent chance.
The Belgian was sold to Wolfsburg at the beginning of 2014, only to reappear in the Premier League 18 months later at Manchester City, where he's turned into one of the best attacking midfielders in the division.
Mohamed Salah
Liverpool have brought the Egyptian winger back to the Premier League for a club record fee this summer, providing further proof that Chelsea might just be a bit too quick to judge a player.
They let Salah go just a year after signing him from Basel in 2014, but he scored 15 goals and made 11 assists for Roma in Serie A last season, and he looks set to shake up Liverpool's attack in the new campaign.
Daniel Sturridge
After life on the fringes of the team at Stamford Bridge, Sturridge seemingly decided that enough was enough in January 2013 when he jumped ship to Liverpool.
You could argue that his injuries meant that Chelsea were right to let him go, but even with them he is still worth far more than the £12m Liverpool paid for him - a bargain considering he has scored 60 goals in 119 games for them.
Ryan Bertrand
After coming through the ranks at Chelsea, left-back Bertrand had to rely on a huge succession of loan moves for his consistent first-team football, although he did play in Blues' Champions League final win of 2012.
Now seen in the red and white of Southampton, he could follow Lukaku in returning to Stamford Bridge this summer.
Nemanja Matic
After joining in 2009, giant Serbian midfielder Matic played just three games for Chelsea's first team before he was shipped off to Benfica as part of a deal for David Luiz in 2011, but just three years later he was back for a hefty £21m.
After winning the league under Jose Mourinho in 2015, he could now reunite with his former boss at Manchester United.
Juan Cuadrado
Now this really was a strange one.
Colombian Cuadrado shone at the 2014 World Cup and looked set to be a huge star at Chelsea when he joined from Fiorentina six months later, but he was barely seen in a Blues shirt before being shipped off to Juventus on loan.
He's been excellent there - the odd Champions League final red card apart - and joined them permanently in May.
David Luiz
Chelsea couldn't really turn down the £50m offer from PSG for erratic defender Luiz in 2014, but when they got him back two years later they were getting a better player.
Playing at the heart of a back three, the Brazilian was excellent in Chelsea's title victory last season, showing that they can benefit from the improvement of their players just as much as others have.