Manchester rivals battle it out for defender Eliaquim Mangala
Mangala, who is also interested in a move to Chelsea, is considered one of the top European central defensive talents. United's new manager, Louis van Gaal, is hedging his bets over who he signs to replace the departed Nemanja Vidic and Rio Ferdinand.
The 23-year-old's temperament is one of the factors which has led to Van Gaal's hesitance, as United prepare to assemble a squad capable of returning immediately to the Champions League.
He struggled last season to recapture the form he found in his first two years at Porto and his substantial €45m (£36.3m) release clause is another potential impediment. Van Gaal's preference is the Borussia Dortmund defender Mats Hummels.
United had hoped to conclude some of their transfer business before the start of the World Cup and that is still thought to remain a possibility, with Van Gaal's commitment to the Dutch national side's preparations for Brazil not precluding him from regular contact with United chief executive Ed Woodward.
City find themselves with the same difficulty finding a central defensive reinforcement this summer that they did last, when they made Real Madrid's Pepe their top target, only to find the Spaniards unwilling to let him go.
Roma's £20m 27-year-old Moroccan Mehdi Benatia is a possible back-up option, though, after a number of central defenders have struggled to form partnerships with captain Vincent Kompany, City know they must get this decision right.