Mikel Arteta will miss the rest of Arsenal's season with an ankle ligament injury.
The Spanish midfielder, who has played an influential role in Arsenal's improved form in recent weeks, hurt his ankle while attempting a cross in the early stages of Monday's 2-1 defeat to Wigan Athletic at the Emirates Stadium.
Arteta received treatment off the pitch - during which time Wigan scored their second goal - but he could not continue. Scans showed the ankle was not broken but the ligament damage was bad enough to keep him out for Arsenal's final four Premier League matches.
Arsenal manager Arsene Wenger, whose side face Chelsea at home in the league on Saturday, now has a gap to fill in midfield as he is without long-term absentee Jack Wilshere and Abou Diaby, who is not match fit.
Yossi Benayoun, on loan from Chelsea, is ineligible to play against his parent club.
Arsenal are third, five points ahead of their arch-rivals Tottenham Hotspur who have a game in hand.
After facing Chelsea this weekend, Arsenal go to Stoke City, then play Norwich City at home before ending their campaign at West Bromwich Albion.