Gabriel Paulista has been taken off with an injury.
The Brazilian was substituted after just 36 minutes in the clash at Loftus Road against QPR, with the score at 0-0.
French centre-back Laurent Koscielny came on in his place, resuming his partnership with Per Mertesacker.
There's a break in play here as Gabriel receives treatment... 0-0 (36) #QPRvAFC— Arsenal FC (@Arsenal) March 4, 2015
Substitution: Laurent Koscielny comes on for Gabriel Paulista 0-0 (36) #QPRvAFC— Arsenal FC (@Arsenal) March 4, 2015
The extent of the injury to Paulista is not yet known.
The defender was bought in January from La Liga side Villareal, with the club's lack of cover at centre-half badly exposed earlier in the season.
He was one of their most impressive players in the win over Everton at the weekend, communicating effectively with Colombian international goalkeeper David Ospina.
Teammate Francis Coquelin is playing despite a broken nose, wearing a protective mask to help him.
Arsenal's next match is the FA Cup quarter-final fixture at Old Trafford on Monday night, and went into the game at QPR in third position in the Premier League.