United boss Sir Alex Ferguson has responded to intense speculation over Rooney's future by insisting the England striker will still be at Old Trafford next season.
But Mancini was still asked whether he might be tempted to test United's resolve over the 27-year-old as he met media to preview his side's FA Cup clash with Barnsley this weekend.
City were heavily linked with a move for Rooney 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.
Mancini says he would be surprised if something such as that led to his departure.
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."