Manchester United have several good defenders, that position shouldn't be their problem but getting a better goalkeeper, Onana made lots of errors last season I wonder whether you didn't notice it, if he was manageable the club would've focus on the defends instead thinking of a replacement.
Onana performance was so poor last season, I was thinking the first thing that Manchester United will do is get a new goalkeeper when the transfer window is open, but they didn’t do that, but seriously Manchester United just need to sign a better goalkeeper, I don’t really know if Onana performance is going to improve, they just need replacement for him, they shouldn’t depend on Onana completely.
Everyone have been complaining about Onana, but I guess we are not suppose to blame only Onana, I think we should blame the defenders also, Manchester United defense is not that strong as you said, there are some balls that are not suppose to get to the goalkeeper, if Manchester United had a strong defense last season, then they will have been able to protect even the goalkeeper, but they have a weak defense, so their opponents are always finding it easy to break.