The focus on fixing the front line has been very good for Manchester United, but they also need to think about the defense because last season we saw that Manchester United conceded quite a lot of goals. Bryan Mbeumo is indeed one of the key players at Brentford because he is a player who always works hard and Bryan Mbeumo's presence can also have a positive effect on Manchester United, but as I said before and Manchester United also need to think about the defense because if the front line is so solid but they have problems in the defense then this can also have a bad effect.
Honestly, I agree with what you said, that Man United's back line also needs serious attention, but Man United seems to want to really build a team without having to rush, for that reason, I think Man United doesn't want to focus on other players before the Mbeumo transfer is clear. There are rumors that Man United are also hunting for a goalkeeper, Emi Martinez and Diogo Costa are the candidates. So with this speculation, Man United also seems quite concerned about their back line.
Brentford are keen to capitalise on this opportunity to make some serious cash, and it looks like Man United will be keen to do so as they are very serious about signing Mbeumo, and it seems there is no other option than Mbeumo.