"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 34 year olds last England appearance was in the 2-2 draw against Switzerland in June 2011.
"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]."