Arsenal boss Arsene Wenger confirmed a hamstring injury for Gabriel Paulista during victory over QPR.
Gabriel, 24, was starting just his third game for the Gunners since making his £11m move from Villarreal.
But the Brazilian had to be replaced by Laurent Koscielny and is now set for three weeks on the sidelines.
Boss Wenger said: "It's a hamstring. I don't know, it's bad enough to come off.
"It's usually 21 days, not years!"