Nathan Ake completed his move to Bournemouth on Friday in a move reportedly worth £20million as Chelsea allowed another young prospect to leave.
The 22-year-old defender spent the first half of last season on loan with Eddie Howe's side and rejoins them on a long-term deal.
Ake is already full Netherlands international and played in Chelsea's FA Cup semi-final win over Tottenham last season.
Could he be another youngster the Blues sold too soon? Here, we look at six players they may regret allowing to leave permanently as quickly as they did.
Since departing Chelsea for £28m in 2014, Lukaku has scored 53 Premier League goals in three seasons, including 25 in 2016-17. If the Blues want him back, they know his price-tag has risen dramatically.
KEVIN DE BRUYNE
Chelsea allowed De Bruyne to leave for Wolfsburg for £18m in January 2014. Just 18 months later, he signed for Manchester City in a £55m deal and his performances are justifying the massive outlay.
Matic was allowed to join Benfica in 2011 as part of the deal which brought David Luiz to Stamford Bridge. Three years later, they splashed out £21m to re-sign the midfielder, who has since helped them to two Premier League titles and is being heavily linked to Manchester United.
Sturridge left Chelsea for Liverpool in January 2013 and, given the market's explosion, has proven to be a bargain at £12m despite his recent struggles with injury.
After a loan spell, Chelsea let Bertrand join Southampton for a reported £10m in February 2015. The defender has since become a full England international and been linked with a big-money move to City.
Mohamed Salah was let go by Chelsea after only one full season at the club. He starred for Fiorentina and Roma on loan before moving to the Italian capital permanently for €15m. Liverpool have just signed him on a deal that could reach an astonishing €50m.