Arsenal head coach Unai Emery says defender Rob Holding suffered "a big knee injury" in the 2-2 draw against Manchester United.
The Englishman was stretchered off nine minutes before half-time after a coming together with United striker Marcus Rashford and the former Bolton man could now face a long spell on the sidelines.
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 his 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.
Arsenal are now 20 games unbeaten following their draw at Old Trafford but dropped to fifth in the table.