Mourinho has expressed interest in Dier and Matic for some time, but Tottenham Hotspur and Chelsea respectively have been reluctant to enter negotiations with their rivals.
Roma have insisted that Nainggolan is not for sale, but the Belgian has refused to rule out the prospect of a move away this season.
And with Ivan Perisic’s arrival seemingly imminent, it seems as if United’s final task in the transfer window will be to sway one of the aforementioned clubs with a bid they cannot refuse.
Negotiating with Spurs, and Daniel Levy in particular, already seems as if it would be pointless. Recent reports suggest that the North London side will not even entertain accepting a £60m bid – £10m more than their initial valuation – for the Englishman.
And Chelsea, with Antonio Conte still sulking following Romelu Lukaku’s move to Old Trafford, aren’t likely to engage in negotiations over any of their players, it seems.
The best option, therefore, comes in the form of Nainggolan: the 28-year-old, who claimed 15 goals last term, will be allowed to leave if Man United make an adequate offer as Roma look to re-build their depleted squad after seeing a number of key players depart.