Heung-min Son gave Spurs the lead against the Portuguese’s former club but Fred, Edinson Cavani and Mason Greenwood scored in the second half as United turned the game around.
Having slipped up away to relegation-threatened Newcastle the weekend before, Spurs have now taken only four points from the last 12 available.
And that has seen them slip to seventh in the Premier League, six points off the Champions League places.
‘There was an element of blankness on Mourinho’s face’
The Times reported last month that Mourinho was running out of support in the Tottenham dressing room, and that a top-four finish represented his best hope of keeping his job.
And Neville feels the manager’s demeanour is concerning.
“I just looked at him in that last five or ten minutes, and there was an element of blankness on his face,” he said on his Sky Sports podcast.
“I just thought he doesn’t look like he’s engaged there. And that would be a worry.”
Neville: ‘I saw it last week with Arsenal’
For Neville, Spurs lacked ‘intent’, much as their rivals Arsenal had done in a dismal 3-0 defeat to Liverpool a week previous.
He stopped short of suggesting that the squad had stopped playing for their manager, but said the atmosphere at the club was a little ominous.
“It’s body language, it’s that intent you want to see in a team,” he said.
“I saw it last week with Arsenal. And I didn’t like what I saw in that second half today.
“That’s not to say he’s lost the changing room, but I didn’t like it. It didn’t feel right.”
Mourinho has a chance to land Spurs’ first silverware for 13 years when he faces off with Pep Guardiola’s Manchester City in the Carabao Cup final in a fortnight’s time.
You suspect that defeat at Wembley would leave his job virtually hanging by a thread.