Louis van Gaal seemed strangely out of character. Chastened, resigned, humbled even, Manchester United’s manager cut a somehow diminished figure as he answered questions about his team’s latest setback.

Whether or not José Mourinho is to succeed him at Old Trafford, a coach who once revelled in being the Pep Guardiola of his day appears to accept the end-game has begun. Gone was the defiance, the bristling in the face of journalistic inquiries and, perhaps most startlingly, the old antipathy towards Sam Allardyce.

After agreeing that, yes, it would be extremely difficult for United to finish in the top four this season, Van Gaal suggested their best route back into the Champions League would be by winning the Europa League before hastily offering a caveat. The standard of football in Europe’s secondary competition is high, he cautioned, lifting the trophy cannot be seen as anything remotely near a foregone conclusion.

Last season he had taken offence when Allardyce, then in charge of West Ham, accused United of deploying long balls and subsequently produced diagrams intended to debunk that outrageous notion. Now, though, Van Gaal was praising an old foe, lauding the controlled aggression, stellar set-piece execution and accurate long-ball delivery of this relegation-threatened Sunderland side before acknowledging United were falling short of minimal expectations.

“The top four will be very difficult now,” he said. “You cannot close your eyes to that. Everybody’s very sad. We couldn’t cope with Sunderland’s aggression and set pieces. We didn’t deliver and we feel disappointed and we feel sad. You cannot close your eyes from the top four being a minimum requirement.”

Seated on a little stage in the Stadium of Light’s media room, Van Gaal looked tired. He seemed in need of some sun on his back and heat in his bones. Instead he must head to central Denmark where United play the first leg of their last-32 tie against Midtjylland on Thursday.

In contrast Sunderland are bound for the bling and bright blue skies of Dubai, with Allardyce now confident the restorative properties of the Arabian sun will be transposed into relegation-averting points.

After succeeding Dick Advocaat in October, he initially struggled to make the desired impact on Wearside but appears to have shopped very well in January. Here the outstanding Wahbi Khazri – a Tunisia playmaker signed for £9m from Bordeaux – scored the opener courtesy of a curling, thoroughly deceptive, 30-yard free-kick and then delivered the corner from which Lamine Koné’s header forced David de Gea into an own goal.

Although Anthony Martial registered a fine, delicately dinked and tightly angled, equaliser, the excellence of Koné – a £5m purchase from Lorient – alongside John O’Shea ensured Vito Mannone remained well protected.

Despite United enjoying a period of first-half dominance, their passing was too slow and complicated to inflict serious damage on hosts who sensibly piled pressure on Van Gaal’s novice full-backs, Donald Love – on after Matteo Darmian dislocated a shoulder– and Cameron Borthwick-Jackson.

With Yann M’Vila increasingly showing off his influential central-midfield poise, Sunderland were deserved winners at the end of an awful week dominated by the sacking of Adam Johnson.

“It was a massive three points,” said O’Shea. “We’re a lot closer to safety. We saw Wahbi’s ability the minute he joined us, he puts in quality dead balls every time. He’ll be a big asset, you can see us scoring a lot more goals from set pieces.”

It was Sunderland’s first home league win against United since 1997 and O’Shea, who spent so many years at Old Trafford as part of Sir Alex Ferguson’s defence, was suitably delighted. “It’s special,” he said.

“Manchester United are still a very good team so we deserve a bit of credit but they don’t have as many players as they used to and they aren’t where they want to be in the table, that’s for sure. Given the budget, the money spent, qualifying for the Champions League is the minimum expected of them, it’s the given. But they’ve lost some very important players to injuries and the quality of the Premier League is increasing, as is the intensity.”

A few yards away Allardyce was savouring the reality that O’Shea and Koné had succeeded in making Wayne Rooney look ordinary. “John and Lamine have struck up a good partnership,” he said. “John will sweep around and cover, Lamine will head anything and intimidates centre-forwards.”

It was suggested Koné’s performance was reminiscent of those once produced in the 1970s and 1980s by a centre-half known as “Big Sam”. Sunderland’s manager laughed. “Lamine’s a bit better on the ball than me,” he said. “My second touch was a tackle.”

Man of the match Wahbi Khazri (Sunderland)