Theo Walcott has been ruled out of Arsenal's Champions League clash with Montpellier on Wednesday night due to a shoulder problem.
The England striker sustained the injury during Arsenal's 5-2 derby win over Tottenham at the weekend but was able to play on and scored the Gunners' fifth goal in injury-time at the end of the match.
Arsenal boss Arsene Wenger revealed Walcott had not trained on Tuesday and would have a scan to determine the extent of the injury although he thought it would be short-term.
Gervinho is back in the squad for the Emirates Stadium tie.
Wenger hopes Walcott's problem will not prove too severe.
He said at Tuesday afternoon's pre-match press conference: "The injury he got during the game with his shoulder, we have to scan that today and see if there is no damage. He will be out for tomorrow. It should be short-term.
"We will miss [Andre] Santos as well, who has a muscle strain in his abdomen. The two players coming in the squad are Gervinho and [Kieran] Gibbs."
Walcott, 23, has yet to agree new deal at Arsenal, and as such could leave at the end of the season on a free transfer.
Wenger has ruled out selling the forward during the January transfer window should the impasse over negotiations not be resolved.
The Gunners boss said: "There's a risk, yes, but that we are ready to take. A successful season is more important for us than this risk that is only a financial risk, apart from the losing the player. If we sell him, we lose him anyway."