yes With Hojlund being absent for 3 to 5 weeks this will be a big loss for Manchester United who are currently in the hunt for 4th place in the standings. In my opinion Erik Ten Hag will again lose stability in his attack line. They will face Manchester City in the 28th match which will certainly only make the situation worse. Even if it is maximized there is still a possibility that Manchester United can at least finish in the top 6 and play in the European League.
3-5 weeks?
It is a really long time period for the Red Devils. They are really unlucky about his injury. As far as I remember he was injured for a while when he first arrived here as well. The fans would hope to see him healthier than this. Especially while there is a big improvement in his performance the team need him a lot. 
About the Manchester City game yeah his absence would make things worse in the attacking line. However I think with or without him the Red Devils would lose this game anyway. I appreciate the way they improved their form recently but this game is still very difficult.