The 21-year-old Moroccan was injured after falling awkwardly 10 minutes from the end of their friendly win over Watford at Stevenage Borough.
Manager Martin Jol said: "We are waiting for the results of a scan but I think he will be out for at least two weeks.
"It's very disappointing news for Mounir because he'd been involved with the first-team squad throughout pre-season."