Aguero has been hampered by problems this season and is now seriously concerned that another setback would rule him out of the World Cup in Brazil.
Demichelis said: "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."