Sir Alex Ferguson says he was "as surprised as anyone" about for the World Cup qualifiers.
But he stressed the 34-year-old will only be released after consultation with the Manchester United club doctor.
"I was as surprised as anyone when I heard," Ferguson said.
"I need to speak to the [United] doctor because we prepare Ferdinand in a certain way and there are treatments he has to go through."
The United defender's last England appearance was in the 2-2 draw against Switzerland in June 2011.
But his England career had looked to be over after he was left out of the squad for Euro 2012 in Poland and Ukraine, with England manager Roy Hodgson citing "footballing reasons" for the omission.
"He has to make sure he is going to be OK in terms of the number of games he plays," Ferguson added.
"We have been doing that for two years and it has worked.
"We have to look at all these things before Rio decides [if he joins up]."
Ferdinand, who has won 81 caps, has been in excellent form for United this season as they sit 12 points clear in the Premier League.
"The procedures have been the same all season which has given him fantastic performance levels when he's playing for us," said Ferguson.
"We have to consider that and Rio is considering that. We'll see what the outcome is."
England manager Hodgson revealed he had not told Ferdinand about his call-up on Thursday.
"I haven't spoken to Rio. I like to do my talking to players face to face," said Hodgson.
"The telephone sometimes is not the greatest of instruments, especially if you don't know people extremely well."
Hodgson also maintained there was no "sinister connection" for his omission until now and said he deserved the call-up on current form.
England play San Marino on Friday, 22 March, before travelling to Montenegro on Tuesday, 26 March in a game which could dictate who wins Group H.