Not saying he will be a star, but it's a sensible loan. We need a striker on loan because we can't buy anyone till the summer. Maybe we could afford Thuram but he is not good enough and would probably demand a 5 year contract. Who else would be available on loan that is better than Weghorst?
Anyway, Burnley was relegated that season. They won 7 matches out of 38, scored 34 and conceded 53. Does that scream like a team that creates a lot of chances? Even Lewandowski wouldn't score a lot of goals in 20 matches for them.
And Weghorst isn't exactly a game changer like Mbappé, Neymar, Rashford, Salah etc. He will be a target striker and needs to be in a team that creates chances, and United creates a lot of them. Very likely he will score many more goals than his period for Burnley.