Rob Holding is expected to miss the rest of the season after his knee injury at Manchester United
The Arsenal centre-back was taken off on a stretcher in the first half of the 2-2 draw.
And the Gunners are set to reveal the 23-year-old has suffered cruciate-ligament damage that could rule him out for several months.
The former England Under-21 star was hurt in a challenge with Marcus Rashford.
And it is understood his boot was unable to grip on the astroturf around the perimeter of the pitch.
Holding has been was in fine form for Arsenal under Unai Emery, sparking talk of a senior call-up for the Three Lions.
Those blows have sent speculation soaring that Emery will look for new defenders in the January transfer window.
Meanwhile, the Gunners dropped to fifth after a point and performance at Old Trafford that Emery was ”proud” of.
But they are still eight points and three places above a United side whose own gutsy display justified the axing of their two costliest players, Romelu Lukaku and Paul Pogba.