Ferguson: Rio undecided over England call-up
The 34-year-old centre-back was recalled on Thursday after a nearly two-year absence from the international squad for this month's World Cup qualifiers against San Marino and Montenegro.
England manager Roy Hodgson has said he would understand if Ferdinand felt a "grievance" towards him, having been overlooked for so long, but Ferguson said the player's concerns were medical.
"I was as surprised as anyone when I heard that [he had been called up]," Ferguson told a news conference.
"Obviously Rio needs to speak to a doctor. We prepare Rio in certain ways, he has certain treatments he has to go through, the procedures have been the same all season which has given him a fantastic performance level when he has played for us.
"So we have to consider that and Rio is considering that and we will see what the outcome of that is."
When asked if it was possible Ferdinand might not meet up with the England squad, Ferguson replied: "I think he has to make sure he is going to be okay in terms of the number of games he plays and that's a factor.
"We've been doing that for two years for the lad. It's worked, the way we've detailed his training programmes to give him the treatments he needs for the various complaints he has in terms of his back and groin and things like that and so far it has worked very well so we have to look at all these things before Rio decides."
Ferdinand, who made his debut for England 16 years ago and won the last of his 81 international caps against Switzerland in June 2011, was snubbed for the Euro 2012 finals in what Hodgson described then as a "footballing decision".
Critics suggested the reason Ferdinand was omitted from the Euro 2012 squad was because of tensions with former England skipper John Terry, who was involved in a racism row with Ferdinand's brother Anton.
Terry's subsequent international retirement however has left England short of pedigree in the central defensive areas.
Hodgson said he had decided to bring back Ferdinand following a series of outstanding performances for United who are closing in on their 20th Premier League title.
Asked whether Ferdinand could have a grudge against him for not being picked until now, Hodgson told British newspapers: "I suppose it is a concern, yes.
"But I can't do more than I am now, giving him the chance to come back, asking him to come back, making it clear I would like to use him. I can't do more than that.
"If he has a grievance and does hold it against me that I haven't selected him previously, then there is not much I can do about that."
England are second in qualifying Group H with six matches to play and face minnows San Marino next Friday before they take on pacesetters Montenegro on March 26.