Obertan joined United in a shock move from Bordeaux for around £3million in July.
The 20-year-old has been undergoing treatment on a back injury since joining the club.
United boss Ferguson said: “Gabriel has had a spine problem but I think he’ll be back in about a month to six weeks’ time.
“We’re taking our time with him because we want him back and at 100 per cent.”
Ferguson added: “He’s very quick, he can play both sides or through the middle and he brings a real versatility to us.
“At 16 he was one of the outstanding young talents in Europe.
“He said to us that over this last year and a half he’s been playing with this back injury and no-one thought about getting a scan.
“We expect big things from him. He’s a nice boy, speaks good English and should have no problem settling in.”