McTominay has thrived in Italy as he's helping Napoli establish a title challenge this season.
"Oh how Manchester United must regret selling Scott McTominay," Souness told the Daily Mail. "He’s a fantastic modern footballer. A high-energy, box-to-box workaholic who chips in with important goals.
"He was 27 years old and not hurting United with his wages when the club’s ‘decision-makers’ thought it was a bright idea to sell him to Napoli for £25million.
"They’d gone out and spent £58m on an ageing, 30-year-old Casemiro, and paid him £370,000 a week. And another £42m on Manuel Ugarte. Collective transfer fees of £100m.
"Would someone at United please hold their hand up and say, “This was my shout.” Because it’s been an atrocious piece of judgment.
"The perfect example of the mismanagement on an industrial scale at that club since Fergie packed in. McTominay has flourished at Napoli. Maybe United should be asking themselves how much it will cost to get him back."