Arsenal manager Unai Emery says defender Rob Holding suffered “a big knee injury” and faces a long spell on the sidelines.
The Englishman was stretchered off in the 2-2 draw against Manchester United after a coming together with United striker Marcus Rashford.
But Emery had better news on Arsenal midfielder Aaron Ramsey, who was replaced at half-time by Henrikh Mkhitaryan.
Emery said: “It’s the worst news for us today. We are going to wait for the doctor but maybe it’s a big injury with Holding’s knee.
“Aaron Ramsey is a small injury but after these two injuries, every player responded with good commitment and good mentality to keep in the difficult moments our performance in the game.”
Holding has made 16 appearances for Arsenal in all competitions this season with centre-back Laurent Koscielny and left-back Nacho Monreal out with injury.