Arsenal manager Arsene Wenger has confirmed that defender Gabriel Paulista will be out for at least two weeks.
The 24-year-old suffered the injury in Wednesday's 2-1 win against Queens Park Rangers and will miss games against Manchester United in the FA Cup as well as Premier League games against West Ham United and Newcastle United.
He is almost certain to also miss the away game against Monaco in the Champions League.
"Gabriel has a little hamstring injury and he will be out for two to three weeks," Wenger said.
"He has a scan today. That means he's not available for the weekend and certainly not for West Ham United or Monaco, but after that he should be available again."
Gabriel is the only main injury concern for Wenger with goalkeeper Wojciech Szczesny to undergo a late fitness ahead of Monday's FA Cup game against the Red Devils.
"Nacho Monreal might come back. He will have a test today," the Frenchman explained.
"Everybody else should be available - the same squad that played against Queens Park Rangers.
"He [Wojciech Szczesny] has a test today. If he gets through that test, he will play on Monday night.
"We have a lot of competition going on in the team, especially up front - we have plenty of offensive potential. It makes every decision more difficult."
