Keegan: Man City defender Nastasic will be better than Man Utd's Vidic
The 20-year-old has been one of the stand-out performers for City this season.
"I really like him," said Keegan. "He has come in and played as if he has been there for ever. That Mancini put him in for his debut against Real Madrid in the Bernabeu tells you everything you need to know about him.
"In five or six years time you could see him developing into a Vidic. He could become even bigger than Vidic, who has probably been the best centre half in this country for the last five or six years.
"The big test will come next season when people have had a good look at him. They will try and find out what he is not good at and where his weaknesses are. I haven't seen too many at the moment."