Arsene Wenger has described Alex Oxlade-Chamberlain's latest injury as "quite serious" and confirmed the winger is potentially facing seven weeks out.
Oxlade-Chamberlain was hurt in a high-speed collision with Barcelona defender Javier Mascherano during the first half of Tuesday's 2-0 Champions League reverse at the Emirates.
He carried on until half-time but emerged from the tunnel with a clear limp and was substituted for Theo Walcott in the 50th minute.
The initial fear was that the 22-year-old had suffered both knee and ankle ligament damage and could even sit out the remainder of the season. However, the Gunners are still waiting on the results of a scan, which will determine whether he will be out for "two to three" weeks or "six to seven".
Wenger, speaking ahead of Arsenal's trip to Manchester United on Sunday, says he simply will not be able to put a timeframe on his return until the results are in. However, his tone suggests he is already fearing the worst.
The Frenchman said: "His injury is quite serious and keeps him out for a few weeks. He is consulting a specialist. He has injured his knee. Hopefully we will have good news, but he's out for a few weeks. How long exactly, I don't know.
"Let's hope that we get good news and that it's only two or three weeks and not six or seven."
The injury occurred when Oxlade-Chamberlain ran full speed towards the Barcelona penalty area during a promising break. The England international overran the ball by a fraction, allowing Mascherano the chance to get it clear. The Argentinian won the race by a fraction of a second but the resulting collision saw both receive lengthy treatment.
Cut in half
On the tackle, Wenger said: "He was cut in two by Mascherano. I don't think it was a malicious tackle. It was a fully committed tackle, but it didn't look malicious."
Oxlade-Chamberlain has been limited to just 17 starts for Arsenal this season and has made a further 16 from the bench. He has two goals in what has been a disappointing campaign but had been rekindling his best form prior to the Barcelona game.
More injury problems
The worst of Arsenal's injury problems appear to be behind them. Francis Coquelin and Alexis Sanchez are both returning to full fitness following long absences while Jack Wilshere and Santi Cazorla are expected back in early March.
And Arsenal could also welcome back Gabriel Paulista in time for the game against the Red Devils. The Brazilian has missed the last three games with a hamstring injury and will undergo a fitness test on Friday and then again on Saturday before a final decision is made.