After striking a deal over a fee with the Saints, Chelsea set about finalising the terms of a move which sources have confirmed is worth around £58m including add-ons.
Southampton director Jason Wilcox confirmed earlier on Tuesday that the two clubs were still in negotiations at that point, but just a few hours later, the Saints have shook hands with Chelsea and are bidding farewell to Lavia.
Lavia is due to undergo a medical with Chelsea in the coming days ahead of putting pen to paper on another long-term contract at Stamford Bridge.
The Belgian had the opportunity to join Liverpool this summer but, like Moises Caicedo, has opted to join Chelsea instead.
Should all go to plan with his medical, Lavia will become the latest in a long list of new signings at Chelsea.
Alongside Caicedo, the Blues have already recruited Axel Disasi, Christopher Nkunku, Nicolas Jackson, Lesley Ugochukwu, Angelo Gabriel, Robert Sanchez and Diego Moreira this summer, and they are not expected to finish there.
Indeed, soccerhuz.com understands Chelsea have agreed a deal to sign Crystal Palace midfielder Michael Olise, who has a £35m release clause in his contract with the Eagles.
The Blues are then expected to turn their attention to a new goalkeeper to replace Kepa Arrizabalaga, who has joined Real Madrid on loan, while club officials have not yet ruled out pursuing another striker as well.
