Although United won 2-0 at Brighton on Sunday's final day of the campaign, that was only good enough for eighth place -- their worst position at the end of a season since they came 13th under celebrated manager Alex Ferguson in 1990.
Hopes of qualifying for Europe next season now rest on United beating Manchester City in next weekend's FA Cup final.
And the result at Wembley could have a major bearing on whether the Dutchman keeps his job after a traumatic season at Old Trafford.
United will have the added incentive of trying to deny local rivals City another domestic double after Pep Guardiola's side clinched an unprecedented fourth successive English title on Sunday.
But even if United lift the FA Cup, there is no disguising the pain of a league season in which the 20-time league champions suffered 14 defeats and finished with a negative goal difference.
United manager Ten Hag thanked United's supporters for their backing after late goals from Diogo Dalot and substitute Rasmus Hojlund ensured Roberto De Zerbi's last match as Brighton boss ended in defeat.
"We have the best fans in the world and I mean this," said Ten Hag, whose squad has been blighted by injuries this season.
"I have never had this experience when you have a season like us, which is by far not good enough, that the supporters are with us."
The 54-year-old added: "They are with us in the bad season and in bad times they are there. I think also they know why the bad times are there.
"But we have to pay them back and we have to do everything to give them a trophy next week -- but also for the future, we have to perform better."
