Manchester United require a serious reboot in the summer after their season ended in concerning fashion.

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This dire defeat to already-relegated Cardiff followed last Sunday’s draw at the also doomed Huddersfield and means they have flat-lined since Ole Gunnar Solskjær was made permanent manager. That was in late March and since then only Watford and West Ham have been beaten in a run of two wins and eight defeats.

Solskjær signed a three-year deal and believes a rebuild is required. He may need all of this time to make United contenders again after they finished a yawning 32 points behind the champions, Manchester City. Their 66 points are 15 less than last term’s 81, when, under José Mourinho, they finished second. It means United end in sixth place – the position they occupied when Solskjær took over before Christmas – and face participation in the Europa League qualifiers in July if Watford win the FA Cup.

Asked if the owners, the Glazers, and Ed Woodward, the executive vice-chairman, understand how difficult United’s position is Solskjær was emphatic.

“Definitely,” he said. “I’ve had loads of conversations and they’ve been up front and honest. They know we’re in for a rebuild. We know where we are and our form towards the end of the season makes it clearer. When we had that great run [eight consecutive wins] and played some great football at times but towards the end we’ve seen that exceptional efficiency didn’t last. The big plus is the season is over.”

Facebook Twitter Pinterest Paul Pogba (right) was subject of abuse by a Manchester United fan after the match. Photograph: Paul Currie/BPI/Rex/Shutterstock

Solskjær admitted he was not shocked at how United have gone into free-fall. “No,” he said. “It’s not a shock.”

While Juan Mata, Ander Herrera, and Antonio Valencia could lead a summer exodus of players, one Solskjær hopes to keep is David de Gea though there is no progress regarding the goalkeeper signing a new deal.

“David’s got one year left and we want to extend with him and we’re in talks and let’s hope it will get sorted and 1 July we know who’ll be with us,” he said. “It’s going to be a very important pre-season for us.”

United finished as close to Cardiff in the final relegation berth as City at the top and had to suffer Neil Warnock, the manager who has overseen the Bluebirds’ demise, saying: “It will take more than two or three windows to turn things around. The fans are going to have to bite their tongues for a few years.”

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Warnock stated he would never manage again in the Premier League even if he guided Cardiff back up instantly next season. He would not be drawn why.

Warnock is, though, to meet the hierarchy. “I am having a meeting with the chairman [Mehmet Dalman] and chief executive [Ken Choo] on Monday. I’m looking forward to it,” he said. “The club is in a strong position from when I took over and they can push on like Burnley did when they went down. I will leave the club in a good state whenever I leave.”

United began brightly but were toothless in front of goal as they allowed Cardiff too easy an outing. Solskjær gave the 17-year-old Mason Greenwood a full debut and made five changes as Diogo Dalot, Andreas Pereira, Jesse Lingard and Chris Smalling also came in. Valencia, the captain, was on the bench for a final involvement in the matchday squad, while Herrera was not included.

Warnock made two changes as out went the injured Víctor Camarasa and Danny Ward (to the bench), and in came Kenneth Zohore and Leandro Bacuna.

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Cardiff’s first goal came after Nathaniel Mendez-Laing sprinted past Dalot. When the right-back tried to recover he stuck out a leg and down went the Cardiff man. As Dalot protested, the referee, Jonathan Moss, gave the penalty, which appeared harsh. Up stepped Mendez-Laing who scored to the delight of the travelling fans’ who sang, “1-0 to the Championship” and, to their former manager, “You’re getting sacked in the morning.”

For the second half Solskjær took off Phil Jones and brought on Anthony Martial. The re-jig here involved Scott McTominay going to centre-back and he was soon conceding a throw in near goal under pressure from Zohore.

It was McTominay’s missed tackle that led to Cardiff’s second. This let Josh Murphy in along the right and with wide open space in front of De Gea he picked out Mendez-Laing who finished adroitly.

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United were veering into shambolic territory and it was Cardiff who could and did strut through the rest of the game. Solskjær’s last move was to take off Pereira and Dalot for Angel Gomes and Valencia, the latter receiving a warming reception.

But this was as content as the home fans felt – Paul Pogba being the subject of derogatory chants by some in the Stretford End at the final whistle.