The only surprise, perhaps, is that Manchester United have kept the title race going as long as they have when they are capable of playing this abysmally. They find themselves 16 points behind Manchester City, the newly crowned Premier League champions. Their goal difference is 31 inferior to Pep Guardiola’s team and the final twist in the story of a one-sided title race counts as a humiliation for a club with their haughty ambitions.

It was a defeat at home by the Premier League’s wooden-spoon team, with the Old Trafford stands barely half-full and the players booed off by many of the people who had chosen not to leave early. Was this really the team who won 3-2 at City in one of the season’s great comebacks? It was only eight days earlier that Mourinho’s players could be seen throwing their shirts into a euphoric away end at the Etihad Stadium. Now they were wandering around like zombies, drenched in a Mancunian downpour, losing against the worst team in the league.

Mourinho called his players the “masters of complicated football” but that was being generous after one of the more startling displays of his two years in charge. West Brom have been submerged in the relegation quicksands since December. Their previous away win in the league came in August and there has been only one other occasion in the Premier League years, against Blackburn Rovers in 2011, when United have lost at home against the league’s bottom club.

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Even when Jay Rodriguez headed in the decisive goal, in the 73rd minute, it was alarming to see the lack of fight from a team, lest it be forgotten, that have won this league 20 times. Sir Alex Ferguson used to boast there was no other side in the world who scored as many late goals as his – but not the current version. United drifted aimlessly towards the final whistle without ever looking as if they had the wit or gumption to save themselves. Thousands of people had gone before the end. “Manchester City, we’ve won you the league,” was the cry from the West Brom fans.

On this evidence perhaps West Brom ought to have put Darren Moore in charge earlier in the season. As it is, they can still be relegated next weekend and their victory will be remembered as an almost freakish result from an otherwise wretched season. Yet a result of this nature must enhance Moore’s chances of being given the job full-time and the only possible mitigation for United was that their opponents appeared to have dramatically improved since Alan Pardew’s departure.

All the same it was difficult to find any plausible excuse for a team that finished with Alexis Sánchez, Romelu Lukaku, Marcus Rashford, Anthony Martial and Juan Mata on the pitch but barely landed a glove on willing but limited opponents. Mourinho said he could “smell” a performance of this nature coming, complaining there had been a noticeable deterioration on the training ground since beating City. His players, in other words, have started choosing their games and, in the case of Paul Pogba in particular, it would have been no surprise if Mourinho picked up the stench of complacency.

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Pogba might have risen to the challenge against City, with two of the goals in United’s second-half comeback, but when he can play with that kind of authority it makes it even more perplexing that he can follow it up by having this kind of stinker. He dawdled his way through 57 minutes and there was no dissent from the crowd when Mourinho decided the team’s record signing should be removed so early. Mourinho was in such a dark mood afterwards it would be no surprise if Pogba loses his place against Bournemouth on Wednesday, or even in the FA Cup semi-final against Tottenham on Saturday.

Not that it was all the fault of one player. At one point Pogba had the ball 20 yards inside the opposition half and was so irritated by the lack of movement from his team-mates he put his studs on top of the ball and held out his arms to protest. Unfortunately, when a player with his uncommon ability can play so listlessly he is not in any position to preach.

West Brom had a let-off in the first half when the referee, Paul Tierney, gave Craig Dawson the benefit of the doubt for a penalty-box challenge that put Ander Herrera on the floor. Ben Foster denied Lukaku with two fine saves and there was another occasion when the United striker had the chance to go through the middle only to be let down by his control. Yet the reality for United is that there was never a part of the match when it became a backs-to-the-wall operation for a team that is bound for the Championship.

The most inviting chance of the first half was the one for Jake Livermore, after 11 minutes, when David de Gea’s one-handed save denied him beneath the Stretford End. West Brom played with a sense of adventure that seemed to surprise their hosts. Rodriguez scored the game’s decisive goal from a badly defended corner and, as the United fans headed away, a party was starting a few miles across this divided city.