The Argentine striker spent two years at Old Trafford, winning two Premier League titles, a Champions League crown, the FIFA World Club Cup and a League Cup trophy.
Despite the success, Tevez chose to leave the Red Devils in order to join their local rivals in 2009.
Mancini said that it is written in the stars for the 28-year-old, who spent three months AWOL, to bag the winner in tonight's clash at the Etihad Stadium.
"I think Carlos is destined to score the most important goal in the history of this club," said the Italian. "This is my belief."
Tevez has scored seven goals this campaign.