Speaking to Casino Hawks, former Chelsea winger Nevin believes Man United let the 28-year-old leave on the cheap, and suggested the West London club should have snapped him up.
McTominay left his boyhood club for Napoli back in the summer of 2024 for a reported fee of just £25million.
The Scotland international has gone on to become a key player for Antonio Conte as his side push for the Serie A title, scoring seven goals and providing two assists across all competitions.
Nevin said: "I think Scott McTominay was worth £50 million before he left Manchester United.
“It was a monster mistake by United. If you asked any Scottish people when he left for £25million, everybody went, you what?
“After what he'd been doing for Scotland after that period he'd had in the team for Manchester United when he suddenly was moved a lot farther forward and showed what he's good at in his best position.”
He also claimed Chelsea should have gone in for him, saying: "I know West Ham wanted him, but I'd have taken him at Chelsea in a heartbeat. Absolutely. I mean, he really has that much quality. Is he worth £50 million now?
“Yes. But he was before. I’m delighted for him. I'm not even mildly surprised at how well he's done in Italy. Not even slightly."