Barthez sustained the injury during the 2-1 defeat against Ajax in the Amsterdam tournament.
Alex Ferguson said: "He is definitely out of the trip to Hungary, and is possibly out for the next two weeks."
The injury is said to be taking longer to clear up than first thought by United's medical staff, a more realistic comeback would be the return leg of the Champions League qualifier against Zalaegerszegi on August 27.
The Guardian speculates that Ferguson could be forced to buy extra cover:
‘Ferguson hopes that Barthez could be back "within three weeks" but he is already seeking another goalkeeper before the transfer deadline of August 31. "If Barthez were not to make a quick recovery, I would have to think about that," he said.’