Roberto Mancini says Robin van Persie could be the difference in Manchester United's bid to wrest the title from Manchester City, with the Italian revealing his frustration over failing to sign the striker three months before his August transfer to Old Trafford.

Since joining United from Arsenal for £24m Van Persie has made an enormous impact, his 14 league goals a significant factor in United's seven-point lead over City. Mancini, whose side host Stoke City on New Year's Day, said: "At this moment Robin van Persie is a very important player for United. He has scored 13 or 14 goals and it is clear he has changed their situation. At the moment, he is the difference between us.

"We were very close to signing him, I know what the reason was but I won't say. It was not about money. We wanted Van Persie because we knew he was totally different from our other strikers. We wanted to improve our team, particularly for the Champions League. We were very close and we were sure it would be very good for us but it didn't happen and now we can do nothing about it. We were very close three or four months before he went to United."

On joining United Van Persie said he had made the choice with his heart. But Mancini said: "I don't know if he listened to his heart. He took his decision but three or four months before that we were there."

Despite Mancini's regret at losing out on Van Persie he is content with his attack of Carlos Tevez, Sergio Agüero, Mario Balotelli and Edin Dzeko. "I am happy with my strikers because last season they scored more than anyone else in the Premier League," the manager said.

Balotelli's future at City has been in doubt because of his erratic form and behaviour. Yet Mancini said Sheikh Mansour, the club's owner, views the forward as asset who can help promote the City's brand worldwide.

"Sheikh Mansour likes Balotelli because he recognises the talent and he exports the name of the City over the world," Mancini told Gazzetta dello Sport. "We need to consider that Mario was made as a major investment [costing £24m], and this is not a club that throws its capital through the window. I think Mario will stay but the future depends mostly on him.

"I love him because for many years we live together. But the professional relationship is another thing and I've told Mario what I need. The time of cheap talk is over."

While Balotelli is back in training after a virus and "could be ready for Watford" Mancini will not now have Abdul Razak, Yaya and Kolo Touré for their FA Cup tie on Saturday after Ivory Coast insisted they depart early to prepare for the Africa Cup of Nations.

Mancini said: "We don't have them against Watford because they have a meeting on 5 January at noon. It is incredible."

Kolo Touré's absence should now allow Joleon Lescott a place in the side after being demoted to fourth-choice central defender, though Mancini accepts the 30-year-old may still want to leave in the January transfer window. "I can understand that he wants to play," Mancini said. "He's an England player. He doesn't play but we can't sell him."

City will not now appeal Samir Nasri's sending off for a head butt in Saturday's 4-3 win at Norwich City, but his absence combined with the club's injury list may persuade Mancini to invest in January. "Probably we can have a problem because we lose three players [to the Africa Cup of Nations], we have Micah [Richards], I don't think he can play until end of February. Jack Rodwell we don't know when he can play. Maybe we can recover Aleksandar Kolarov in two weeks," Mancini said.

"We can't sell players. We don't have enough players to play if we sell players."