The veteran shotstopper faces two weeks on the sidelines after suffering the injury during training prior to Saturday's 2-2 draw with Peterborough United.
"We were doing some circuit work and Mark hurt himself stretching to control the ball shortly afterwards," physio Marc Czuczman told the club's official website.
"He opened up an old injury, but on first inspection it seems like there's only a grade one tear. We have started the hamstring protocols. Mark will probably be missing for another two weeks."