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.
Atletico Madrid appear to have taken a step forward in their pursuit of Paulinho.
Atletico's impressive league standing and status as favourites to claim the Europa League title somewhat flatters to deceive onlookers regarding their patchy recent form.
Sporting still offered glimpses of danger too and Montero might have done better when the ball dropped kindly in the middle of the area, but he could only drill straight into the grateful arms of Oblak.
Sitting in second spot in La Liga behind Barcelona, Simeone's men are a playing below their level in the second tier European tournament.
The 1-0 midweek defeat in Lisbon made the tie far from comfortable for Simeone's team and showed that they can be at least partly outmanoeuvred in times when the stakes are high. The side drawn first will play the first leg at home.
French club Marseille will play FC Salzburg, the club backed by soft drinks giant Red Bull, in the other semi-final.
James Benge of the London Evening Standard named the 25-year-old as a target for the Gunners last month, while RMC (viaAS) also reported in March that PSG are willing to trigger his €100 million buyout clause (roughly £88 million).
The first legs will be played on Thursday, 26 April, and the return duels on May 3.