Goals from Marcus Rashford (2), Anthony Martial (1) James (1) gave Ole Gunnar Solskjaer’s side all three points against Chelsea.
Lampard had said before the match that Willian is yet to regain full fitness level having won the 2019 Copa America with Brazil last month.
But Mourinho, a former Man United and Chelsea coach, believes Lampard should have featured the Brazilian international against the Red Devils.
“If William is injured… I understand that Willian is not here, if Willian is not injured and just late for his preparation, I don’t understand why he’s not here,” Mourinho told Sky Sports.
“Because to play 20 mins, to play 30 mins you don’t need to be fit.
“You can come from three weeks with an injury and arrive, if you have the right mentality, you are ready.”
