Anotnio Conte welcomed his Premier League champions back to the club's Cobham training base today, though Costa was absent as he looks to seal a return to the Spanish capital.
The two clubs have been in talks for some time but they are yet to come to an agreement over the 28-year-old.
The deal was expected to be quite striaghtforward given that the player wants out and manager Conte doesn't want to keep him, though it has been complicated by Atletico's ban on registering new players and the differences between the two sides' valuations of the player.
Conte, of course, messaged the Brazilian striker at the end of last season to thank the player for his efforts last season and to inform him that he was not in his plans for the forthcoming campaign.
That's despite Costa finishing last season as Chelsea's top scorer with 22 goals in 42 games in all competitions.
Both sides will be hoping to agree a deal before next Monday, when the Chelsea squad flies out to China and Singapore on tour.
Though 'The Guardian' reports that there are still a lot of compromises needed to be made, with Atletico's first offer of £26m, falling well short of the figure Chelsea had in mind.
New signing Antonio Rudiger will also be absent from Chelsea training this week after he was given extra time off following his involvement in the Confederations Cup.