According to the Daily Star, United are planning to hijack City's move for the talented defender, who Porto value at around £30m.
United have lost Rio Ferdinand and Nemanja Vidic this summer, and questions remain over Chris Smalling and Phil Jones at the position. As a result they've been linked with a number of centre-backs from across Europe's top leagues.
Mangala has often been mentioned as a target, but United's interest appeared to cool after it looked like City were ready to outbid them for the French international.
City were prepared to spend the £30m to sign him, but it looks like United have been informed that Mangala is waiting for an offer from the Old Trafford side before making his decision.
United are also looking at Borussia Dortmund defender Mats Hummels, but they are set to make Mangala their top priority instead. If City miss out, the report claims they will turn their attention to Roma's Mehdi Benatia.
One Porto star who will most likely will be heading to the Etihad Stadium is Fernando. The defensive midfielder is close to agreeing a move to join Manuel Pellegrini's reigning Premier League champions.