No, Man Utd don't really need an additional defender, as they already have enough players in that area. The only player they need to add to their squad is a goalkeeper and that what they should be thinking about now and starting to accelerate their goalkeeping signings before the transfer window closes.
I would say that Manchester united's defense lineup needs to change. Yes , you are right that Manchester united also needs an experienced goalkeeper. Anyway , we have not seen good performance from Manchester United's defenders in the last season. Luke shaw, Maguire, De ligt and Yoro are all quite talented. Manchester United has spent a large amount of money to buy these defenders. Manchester United has spent around 200 million to buy these three defenders, Maguire, De Ligt and Yoro. But even then we are not seeing good performance from Manchester United's defense. However, this may also be due to tactical weaknesses.