William Gallas could miss the rest of the season after suffering a knee injury during Arsenal's UEFA Champions League clash at Villarreal.
Italian Giuseppe Rossi fell into Gallas after he was fouled by Alex Song on the edge of the box, but he trapped the Frenchman's leg underneath him.
Gallas could not continue and was forced off, and now there are real fears he has sustained knee ligament damage.
"Gallas has a medial ligament injury," Wenger told Canal Plus.
"The initial diagnosis doesn't appear to be very positive, but it will be clearer on Wednesday."
With the second leg a little over a week away, Gallas will be absent from the Emirates with the tie delicately poised at 1-1.
Another major doubt is goalkeeper Manuel Almunia, who was forced off after he was also, coincidentally, caught by Rossi.
The Spaniard hurt his ankle and tried to play on but was unable to do so and was replaced by Polish star Lukasz Fabianski.
"As for Almunia it is the second time he has twisted his right ankle, and generally that means a three-week lay-off," he added.