The west London side have been plotting a sensational move to re-sign the Portugal forward who scored four goals in 20 appearances for the club in 2023.
The Blues have reportedly been discussing a £40million fee plus add-ons for the 24-year-old who played for Barcelona last season following another loan move.
If they reach an agreement, Chelsea will look to revive Conor Gallagher's sale to Atletico.
Gallagher, also 24, had been on the verge of joining Atletico this summer and even travelled to Madrid, but complications surrounding Samu Omorodion's proposed transfer to Chelsea saw the deal stall at the last minute.
As a result, the English midfielder found himself on a flight back to London with his future unclear.
But Omorodion has been excluded from Diego Simeone's squad for tonight which was announced this afternoon.
The 20-year-old was expected to finalise his £34.5m move on a seven-year contract with an option for a further year after heading to London last week but negotiations hit a hitch.
The forward featured in Spain's Olympic gold-winning team this month after a successful season which saw him make 35 appearances on loan at Deportivo Alaves, scoring eight goals.
On England international Gallagher, Chelsea manager Enzo Maresca said: 'Conor is back. He is training apart [from the rest of the group].
'He is doing some medical tests and some training.
'He won't be involved in the game. At this moment, he is trying to find a solution with the club.'
Meanwhile, Chelsea defender Bashir Humphreys is set to have a medical at Burnley on Monday ahead of a move to Turf Moor on a season-long loan.
Meanwhile Samu Omorodion, another name on Enzo Maresca's wishlist, has been left out
Felix, 24, was loaned out to Barcelona last season but he has since returned to Atletico Madrid
The Blues kicked off thier Premier League campaign with a 2-0 defeat at home against Man City yesterday.
The champions did not have to be at their best to secure a comfortable victory as goals from Erling Haaland and Mateo Kovacic proved enough to sink Chelsea.
Enzo Maresca's players were committed and united but, ultimately, not quite good enough, particularly when it came to creating telling chances. They face Wolves at Molineux next as new signing Pedro Neto prepares to face his old club.