The Serbian was allowed to move to the Premier League title rival in a deal worth £40 million during the summer and has made a bright start to life at Old Trafford under Jose Mourinho.
Chelsea manager Antonio Conte has since admitted Matic’s departure is a “great loss” to his squad and has previously hinted the decision was not one made by him.
Wilkins, a former Chelsea player and assistant manager, also previously questioned the Blues’ purchase of Tiemoue Bakayoko but ranks Matic’s sale as the worst the club has ever sanctioned.
“It’s the worst I’ve ever seen, if I can say,” he told Sky Sports. “I cannot see that for the life of me.
“You’ve got a guy here who was so paramount to Chelsea along with [N’Golo] Kante in the middle of the pitch, winning the championship for Chelsea the year before and all of a sudden you’ve sold him to one of or the biggest rival we have.
“I say we because I am a Chelsea supporter so I’m speaking from the heart when I speak.
“I just could not see why we would let Nemanja go there. He’s been outstanding for us and you’ve seen in the short period that he’s been there, how he’s allowed [Paul] Pogba to basically go and do what he wants.
“I just couldn’t see that piece of business, at all.”