The 26-year-old midfielder is known for his tenacious playing style and has been praised for his robust nature in the middle of the pitch for the Rome outfit.
Reports claim that United had scouts assessing him over the weekend with Louis van Gaal looking to add depth to his star-studded squad this summer.
Nainggolan appears to be on the verge of a big move this summer as neither Roma nor Cagliari – who co-own him – can afford an outright deal, which has alerted Liverpool.
However, the Red Devils may swoop in and beat their foes with a deal for the £29m-rated Belgian.
The tough-tackling midfield general played a key role during the back end of Roma’s impressive 2013/14 campaign and has been impressive once again this term.