Ten Hag needs to be replaced. He had problems with some of the players in last season like Cristiano Ronaldo. Ten Hag did not play Ronaldo regularly who was one of the top scorers in the season before Ten Hag came to Manchester.
Another Problem with Manchester United squad is that player's performance deteriorates as soon as they join the club. Onana was an amazing Goalkeeper but he has been mediocre at best this season. Although Ten Hag managed to win the coach of the month in the premier league, their performance in UCL has been lackluster. They placed 4th in a group where there were teams like Copenhagen and Galatasaray. They could not even be managed to qualify for the European league. I don't see any solution for Manchester United other than sacking Ten Hag and getting a new coach who could perform with the current squad.
If they sack Ten Hag and get a new coach they might not have the desired results. I think sacking him now is gambling because a new coach will have it start building the club from the beginning. And I also think that Manchester United will get the best coaches at the end of the season. So they should be patient with the coach till the end of the season and if Ten Hag can manage to get the support of the players they might end up getting a Champions League slot this season.
Many United fans keep blaming Onana for the poor results of the club because of his constant mistakes. But they fail to understand that a weak team will make the goalkeeper to be prone to errors. A weak midfield and defence will open up the goalkeeper to constant attacks which will affect his confidence. I think Onana will regain his form if the club fixes all its loopholes.