Atletico Madrid have confirmed that Diego Costa suffered a hamstring injury in Thursday's Europa League match against Sporting CP.
Costa followed shortly after the restart after the striker appeared to sustain an injury to his left leg, but 2010 and 2012 winners Atletico were able to cling on.
Atletico, the 2010 and 2012 victor, will travel to London for the first leg against Arsenal, which has never won the Europa League or the UEFA Cup but did win the 1969 Inter-Cities Fairs Cup a predecessor competition not recognized by UEFA.
But Simeone was delighted with his side's efforts to scrape through to the last four, as they look to reach a fifth European final in nine years.
The Europa League remains Arsenal's salvation this term as it is their only chance of winning silverware, and also because it is their only chance of qualifying for the Champions League given their poor domestic form.
Sitting in second spot in La Liga behind Barcelona, Simeone's men are a playing below their level in the second tier European tournament.
In previous rounds the Spanish side have run to comfortable victories, often finishing the tie by the first leg with hefty victories.
The Gunners came from 2-0 down to draw the second leg of their quarter-final at CSKA Moscow 2-2 on Thursday night, progressing 6-3 on aggregate against the Russian side.
Marseille will host underdog Salzburg in the first leg of the other semifinal.
Extra incentive for Marseille - the final is in Lyon, France.