Gabriel: Arsenal defender out of Man Utd tie with hamstring injury
Arsenal defender Gabriel Paulista has been ruled out for two to three weeks with a hamstring problem.
The Brazilian, a £16m signing from Villareal, was injured in Wednesday's 2-1 Premier League win at QPR.
Arsenal travel to Manchester United on Monday in the FA Cup quarter-final.
"It's a special intensity because it's between two teams with a chance of winning," said Gunners boss Arsene Wenger. "We are the holders of the FA Cup and we want to keep it."
The two sides are separated by one point in the Premier League, with Arsenal in third and United fourth.
Goalkeeper Wojciech Szczesny (illness) and defender Nacho Monreal (back) face fitness tests on Friday but Wenger is hopeful both will be fit.
Ozil 'works much harder than style shows'
Wenger defended playmaker Mesut Ozil after former United midfielder Paul Scholes claimed the German, 26, was only "going through the motions" at Arsenal.
The Frenchman said it was a "wrong statement", adding: "I think a player like Paul Scholes would have loved to play with Mesut Ozil.
"He puts a lot of effort in to help the team and he works much harder than his style shows.
"You can be cheated a little bit by his style of play, because he is fluent, easy, subtle and he does not look like he puts the effort in, but he does."
'Very happy' with Welbeck
Wenger also said former United forward Danny Welbeck has a "great future" at Arsenal.
The 24-year-old has recently played on the right of a front three featuring Olivier Giroud and Alexis Sanchez, rather than through the middle.
Welbeck, a late substitute in Arsenal's last two league matches, could face his former club at Old Trafford on Monday, live on BBC One and the BBC Sport website (kick-off 19:45 GMT).
"I am very, very happy that I bought him and I think this guy has a great future.
"Welbeck is very important and has played many, many games since the start of the season. His position is one of the three up front - central, left or right. He can play anywhere."
Wenger on Arsenal's poor record at Old Trafford
The Gunners have not won at Manchester United since September 2006, but Wenger believes his side are strong away from home.
"I don't believe too much in history, I just believe in the performance on the day," he said.
"At the moment we are doing very well away from home. We are confident from our Premier League run, so we go to Manchester United to qualify and to give absolutely everything."
Wenger on managerial touchline confrontations
"Ideally you want us all to behave in a perfect way. Maybe you have to build a wall between the two dugouts.
"The bigger the distance, the better it is. It's much better at the Emirates - we have zero incidents because we are far apart from each other.
"What happens when the dugouts are very, very close is that everybody speaks to the fourth official and it very quickly becomes controversial."