Arsenal [Images] striker Robin van Persie will miss the first leg of their Champions League [Images] semi-final against Manchester United [Images] with a groin strain.

"He is definitely out for Sunday (against Middlesbrough) and for Manchester United," manager Arsene Wenger told the club's website (www.arsenal.com) on Thursday.

Dutchman Van Persie's absence is a big blow to Arsenal who are unable to use in-form Russian Andrei Arshavin who is cup-tied.

Better news for Arsenal was that first-choice goalkeeper Manuel Almunia is back in training after an ankle injury along with defender Johan Djourou.

Left back Gael Clichy will not recover in time from his back injury to face United next Wednesday at Old Trafford.

"He could be back for the second Manchester United game. He has a scan on Monday. We'll go from there. He has a chance to be back in full training next week," Wenger said.

Central defender William Gallas [Images] will not play again this season after injuring knee ligaments.