United slumped to a second successive defeat as West Ham cruised to a 3-1 Premier League victory at London Stadium to pile more pressure on Mourinho at the culmination of a difficult week, FourFourTwo reported.
Manchester United recorded their worst league start in 29 years.
Felipe Anderson flicked the host ahead early on despite appeals for offside against Pablo Zabaleta, with Victor Lindelof's own goal then making matters worse before Marko Arnautovic wrapped up the win after Marcus Rashford had pulled one back.
And despite claiming that he has no concerns over the attitudes of his players, Mourinho – who took Paul Pogba off during the second half – said United's squad lacked quality.
"I have no complaints with the players' attitude, I can have complaints with some quality and the mental approach in certain duals," he told BT Sport.
"Where you could see for example, [Robert] Snodgrass coming to the pitch with an incredible attitude that he wants to eat the ball and eat everyone around him.
"You need that humility. You have always to try. That's my nature as a football professional. There are certain qualities that you have or you don’t have.
"I think a team that comes with that mental fragility, after the bad result [against Derby] and to come here and in five minutes to be 1-0 down, is not good.
"No VAR and obviously a linesman [makes a] mistake and a goal. Then in the first half we had a good reaction but an own goal makes it 2-0.
"An own goal when we know that [Andriy] Yarmolenko is very left footed. We know we have to cover that side. We are not aggressive enough to stop that shot and we are losing 2-0."
Former United defender Rio Ferdinand said the club faced some "big decisions" over Mourinho's future after the Saturday defeat.
"As a footballer your DNA is about hard work and effort," said Ferdinand, now a pundit with BT Sport.
"I didn't see that today," added the former England center-half, who began his career at West Ham.
"I didn't see them make two or three passes in succession – that's criminal.
"I don't see enough players working hard to say to the manager 'this is my place in the team'. You've got to work hard and grind and Man Utd didn't."
And a concerned Ferdinand insisted, "There are some big decisions to be made at United now. There will be conversations at the top level, about the future of the manager and the squad, because the basics are not being done. There has to be something said."