The 29-year-old joined United from Arsenal in August and has already netted 14 goals in 20 Premier League outings for the Red Devils.
Mancini confirmed that Manchester City tried to pip United to the forward’s signature in the summer and the Italian feels that the Dutchman has already had a significant impact on the form of his new employers.
“Robin van Persie is a very important player for United,” Mancini, whose side are seven points behind the Red Devils in the Premier League table, told BBC Sport.
“He has changed their situation. He is the difference between us.
“We wanted Van Persie because we knew he could be an important player. He is totally different from our other strikers. We wanted him for the Champions League and for the Premier League.
“We were very close. We were sure he was for us but this did not happen and now we can do nothing. We were very close three or four months before he joined United.”
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