The Frenchman completed his move to the Spanish giants in January 2009, bringing an end to an 12-month stay at Fratton Park.
Other notable departures over the years have seen Glen Johnson join Liverpool for £17.5m as well as Jermaine Defore’s £15m exit to Spurs.
But how does the Blues’ record £20m sale compare to their rivals?
We’ve taken a look at the most expensive departure from each League One club to see where Diarra’s figures rank.
Here’s what we found.

13. Port Vale - Gareth Ainsworth
Estimated fee received: £2m; Departed to: Wimbledon; Year: 1998. Photo: Nathan Stirk

14. Burton - Jackson Irvine
Estimated fee received: £2.5m; Departed to: Hull City; Year: 2017. Photo: Nathan Stirk

15. Oxford United - Kemar Roofe
Estimated fee received: £3m; Departed to: Leeds; Year: 2016. Photo: Pete Norton

16. Barnsley - Ashley Ward
Estimated fee received: £4.5m; Departed to: Blackburn; Year: 1998. Photo: Ross Kinnaird

