If faced with a choice I think Manchester United need a defender, I'm not saying the current players are bad but it's not a bad idea to prepare a better squad. Last season's experience should be an example of the failure of the defensive line even though there are many reasons there. Based on Man United transfer records this season, they are prioritizing the attack line by spending 150 million while for their defense line they only spent 4 million + new players from the academy. I don't understand why the coach is so confident in the current defense line or maybe he has other plans?
In my opinion, what they need is a midfielder. In fact, I believe they should buy more than one midfielder. If they can manage to get a central midfielder who is creative, presses well, knows how to retain the ball under pressure and is physical, they can have a good midfield.
People usually underrate a good midfield. No matter how good a defence is, they will struggle if they have a bad midfield, and the attack won't function properly if the midfield is poor.
Currently, in Man United midfield, the only reliable player is Bruno, and he is an AM. If they add a reliable defensive midfielder and a CM, they might be ready.