Cesc Fabregas, the Arsenal captain, will miss this weekend's FA Cup quarter-final trip to Manchester United, Arsene Wenger, the manager, said.
The Spanish midfielder tweaked a hamstring injury during Tuesday's Champions League defeat in Barcelona.
"He will be out maybe for the next two games," Wenger told a news conference.
Wojciech Szczesny, the goalkeeper, suffered a finger injury against Barcelona and will also miss the game at Old Trafford on Saturday.
"He is out for a while," Wenger said.
Robin van Persie, the striker who was controversially sent off against Barcelona, will be available for the United match and Wenger said he would play his strongest team.
"We can take confidence and belief from the Barcelona tie," said Wenger, whose team lost 3-1 at the Nou Camp after winning the home leg 2-1.
Arsenal, three points behind the leaders United in the Premier League with a game in hand, lost to Birmingham City in the League Cup final two weeks ago and the defeat by Barcelona leaves them with only two chances to end a six-year trophy drought.
Meanwhile, Sir Alex Ferguson, the Manchester United manager, has said Arsenal are his team's biggest threat this season.
United lost successive matches in the league to Chelsea and Liverpool to boost Arsenal's title ambitions.
"Arsene Wenger and his players have rearranged the pecking order somewhat to come storming through to lie in second place," Ferguson said. "Last season they faded around the February to March period to leave Chelsea and ourselves in a two-horse race that saw us pipped at the post by a point.
"I think most pundits expected to see something similar this season, plus a challenge from Spurs and Manchester City, to make it even more difficult for Arsenal to get back up to the top."
But that hasn't transpired.
"Clearly they represent the main danger to us achieving our ambitions in a revival of the rivalry of a few years ago, and I am sure they regard us as the major threat to their hopes," Ferguson said.