And now Atletico have confirmed that he has injured his left hamstring, which typically takes at least a fortnight to recover from - which would rule him out of the first leg at the Emirates.
Meanwhile, Diego Simeone's side ran out 2-1 winners on aggregate as they defeated Sporting CP over the two legs.
Arsene Wenger's worst nightmare of the Europa League has just been turned to life. Quique Sánchez Flores won the first Europa League in 2010 against Fulham and in 2012 he won the celebration of ' Cholo ' against Athletic.
Extra incentive for Marseille - the final is in Lyon, France.
It starts at 11am (UK time) and is being held at the Uefa headquarters in Nyon, Switzerland. The Vélodrome will be a boiler in the semifinals as this Thursday against Leipzig.
The Gunners will know that they cannot afford any more wobbles if they are to emerge victorious, with Atletico set to provide a stern test of their credentials.
When are the Europa League semi-finals?
The first legs will be played on April 26, with the second legs on May 3.
Is there a draw for the Europa League final? The x-factor is Diego Costa.
He might be back in time for a potential final, but he won't make the semi-final, so Mesut Ozil, Jack Wilshere and Alex Iwobi really need to step up.
Atletico are the favourites to claim a third Europa League crown after progressing to yet another European semi-final. There is still a lot of work to do until the end and you cannot underestimate clubs like Chelsea, Arsenal, big clubs that can win every game.