Manchester United's defense is quite weak. And this is their biggest weakness this season. Manchester United management should have taken the necessary measures to strengthen the defense. But Manchester United has not bought any experienced defender.
I don't think Manchester United need to sign a new defender this summer. They've got quite a good number of defenders. The defenders just need to stay fit and be injury-free. What they do need is a goalkeeper and a midfielder to make the team solid. However, with the transfer window coming to an end in less than two weeks, I'm not sure they can make it happen. It's most likely they would make do with what they've got and continue reinforcement in the next window.