The defender believes that the injuries have been affecting the 25-year-old as he prepares for Argentina's World Cup campaign.
Demichelis told The People: "Kun is a little bit afraid. He has this fear inside his head because of all his injuries and with the World Cup fast approaching.
"But he probably arrives at the World Cup in his best physical form and we all know that when he is physically OK, he makes a difference on the pitch.
"The guy takes care of himself and is really careful, but you can't know if you are going to get injured or not."
City face Crystal Palace in the Premier League later today, as they look to keep their slim title hopes alive.