Arsene Wenger confirmed Aaron Ramsey suffered a hamstring injury during Arsenal's 2-0 win over Bayern Munich .
The midfielder had to be withdrawn in the second-half after pulling up while closing down a Bayern defender.
While Wenger didn't put a time frame on Ramsey's recovery, he confirmed the player will miss Saturday's clash with Everton.
"He's alright but very down. He told me his hamstring is a very tough one," Wenger said.
"He's out [of the match against Everton], yes."
Realistically, Ramsey, who has struggled with hamstring injuries throughout his career, is facing at least a month out of action.
The Welshman missed large spells of last season with hamstring problems but had cemented his place in the team in recent weeks.
Ramsey's injury could open the door for Alex Oxlade-Chamberlain to start against Everton.