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.
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?
You are right what they need now is a good defender that will serve as a backup in case something went wrong and even if something didn't went wrong they can still use them to start up. Manchester united really suffered a lot last season because of defenders injury especially Martinez injury so the best preventive measure now is to get new defender(s) because I believe the front line is okay as it stands but they need to work on how create good and big chances and also how to convert those big chances to goal because playing well without a win is useless, imagine after Manchester united dominated is Arsenal's match and later lose I mean this doesn't make sense.