However, Ferguson did not rule his captain out of Tuesday's Champions League Group F match against Celtic.
"He trained this morning, but obviously the injury has been a concern," Ferguson told a news conference.
"We are holding our breath...we will have to take a long-term view and do something about it which could mean surgery.
"Had it just been one calf we would be saying to ourselves to give it time, but because he's getting it (pain) in the left one as well it suggests it's something different. Gary is our captain and with his experience we want him in our team all the time."
Victory in Tuesday's match would secure United a place in the last 16.
