Speaking on his programme, The Wayne Rooney Show, the former Manchester United striker reflected on McTominay's departure and the qualities he believes the Scotland international brought to the squad.
"I struggled to understand why Manchester United decided to let him go. He's always been an extremely generous and hard-working player."
Rooney had nothing but praise for McTominay's attitude during his time at the club.
"At the point he was sold, he was one of those players you'd want to see on the pitch every week, especially because he always gave everything, unlike others who often seemed not to be fully committed. His presence was essential for the team."
He pointed specifically to McTominay's role further forward in the side, where he often produced important contributions.
"He was used in a more advanced position and often managed to score decisive goals."
Rooney recalled his reaction when the move to Italy was confirmed.
"When Manchester United chose to sell him, I asked myself: what is going on here?"
The former England captain went further, suggesting he would welcome a reunion between the midfielder and his former club.
"Today, honestly, I'd be happy to see him in a Manchester United shirt again."
McTominay has become a central figure for Napoli since his move from Manchester United, with his form in Serie A also earning him a recall to the Scotland squad for this summer's World Cup, where he has continued to influence matches from midfield.
Rooney's comments add to a growing chorus of former teammates and pundits who have questioned the wisdom of United's decision to let the Scot leave, particularly given the form he has shown in Italy and the impact he has had for his country on the international stage.