City were heavily linked with a move for the 27 year old during the player's infamous contract stand-off with United in 2010.
Mancini said: "I think that Rooney is one of the best strikers in Europe but I think it is difficult he can leave Man Utd - I don't think (so)."
Suggestions Rooney could leave United have gathered pace since he was omitted from United's starting line-up for their Champions League loss to Real Madrid on Tuesday.
The Italian said: "I don't know. I don't know this situation.
"I can understand maybe Rooney would be upset if he didn't play but these are things that can happen in every team. After one week it is finished."