Since joining from Manchester United in 2024, the Englishman has been instrumental in Napoli’s Serie A triumph, earning MVP honours and a Ballon d'Or nomination.
The Scotland international has scored 26 goals in 76 appearances, including a stunning strike against Cagliari, but Nainggolan insists he lacks the refinement required at the highest level.
"He scores a lot of goals, but in general play… he'll always get 12, 13, 14 goals, but if he has to build play, he doesn't have the technique to operate between the lines. He's an average player," he told Sky Calcio Unplugged.
"At my peak I was better than him (McTominay) and Hakan Calhanoglu, but I put Nicolo Barella above myself.
"He doesn't always score many goals, but when he does, they are important, and you can always feel the difference when he plays.