Robin van Persie provided Manchester City with a big 'what-if' moment on Sunday when he netted the winner for Manchester United against the club who wanted to sign him.

The Dutchman's stoppage-time free kick gave United a 3-2 victory in the Manchester derby that sent them six points clear at the top of the Premier League and was the 11th league goal of the season for a man who has transformed their attacking threat.

There could not be a starker contrast than the sight of Van Persie celebrating yet another goal compared to the moody departure down the tunnel of City striker Mario Balotelli who was substituted early in the second half.

The Italian's efforts, including a poor shot way over the bar when he had time to do better in the opening minutes, were greeted by harsh words from manager Roberto Mancini who by contrast spoke of his admiration for Van Persie.

"I love Mario as a guy, also as a player but I think it is important for him to start to think (about) his job if he wants to play well," Mancini told a news conference.

"He can't continue to play like today, we wanted more from him. When you have a player with the quality that Mario has, you can't understand that (he) continues to throw (his quality) out of the window. This is incredible.

"I saw that (type of) player in my life, with fantastic quality and in the end they did nothing. I don't want Mario to finish like these players because it will be no good for him."

He added that City had not lost the game because of Balotelli - although the arrival of Carlos Tevez in his place triggered a change in tempo as they pulled two goals back - but pointed to Van Persie.

"Van Persie (has) scored against every team," Mancini told a news conference. "After United bought Van Persie they improved a lot but now we can do nothing, we have our strikers and I am sure we can win the double this year, also after this defeat."

City's desire to sign Van Persie before his move from Arsenal to United was well known but he picked the 19-times English champions instead and has been instrumental in their charge to the top of the table.

Where City were leading the way last term with their free-scoring ways, it is United who are doing it this time with 40 goals.

"City always come back, so we knew that, and it's a really special way to end the game: with the three points in a romantic way," Van Persie told Sky Sports.

"Wazza (Wayne Rooney) asked me: 'What are you going to do? Are you going to shoot or cross it?' I said: 'I think I'll have a shot'."

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