However, he has returned to training this week and could feature in the Manchester derby at Eastlands on Sunday.
"Dimitar started training today but hamstring injuries are never easy to deal with," United boss Sir Alex Feguson has said.
"We have to be careful because he has never had one before.
"We will have to gauge whether it is wise to use him from the start on Sunday or better to have him on the bench.
"It really depends how he comes through the next couple of days."