Most of you are missing a very important point I feel. It is not only about pace or heading or tackling. Defending is team work. We need a leader in the defence as someone who can organize it. Maguire is zero in that. yes he is a very good header of the ball, he can block, can dribble a bit too but he cannot organise anything. This is one of the biggest problems we have along with the lack of pace. By being the Captain it makes it worse because he does not listen to Lindelof who is a much better organiser than Maguire.
To make matters worse we have a slow coach of a DM too. Fred and McTominay are simply not good enough to be a DM on their own.
If we can get two CBs and a DM who can defend on their own without much input from the other 2 midfield players and our three forwards we can be the champions. Our forwards and midfield are good enough to be creative and score goals on their own if only our defence can hold up on their own.
Zero is harsh, but this statement in substance is not wrong unfortunately. We have a serious issue at the back. I think we only kept 3 clean sheets in the league this season which is a terrible return for a team trying to present a case to become champions. The main problem is, like you said, missing a leader at the back. Maguire, who for some reason beyond me is our captain, does not hold this attribute. He does not command authority at the back. I feel Lindelof is more vocal at times. The pair is fine, but not good enough. Probably we'd need 1 CB to replace Maguire/Lindelof. A CB that can show authority in the box and organise us at the back.
I also agree we need a DM. I like Matic, but I was shocked he got his last contract extension at that age. Clearly his best years are behind him. We need someone who can just sit there when our other 2 midfielders run forward. Someone than can sit nicely in front of the defence. A Kante/Ndidi sort of player. Fred and Mctominay have both been good and will be much needed for rotation/tactical changes. After all, we need depth. We've always had at least 2 good reliable players at each position. So having them would be essential and they will continue to play lots of games, especially when tactically we need 2 defensive midfielders.
I don't see an issue with Wan-Bissaka (who is young and developing) and Shaw/Telles. Probably sorting out our wingers situation would contribute positively to our full backs' defensive display (I'm sure Wan-Bissaka would be much relieved to have a proper right winger in front of him. He can then focus more on his defensive responsibilities and avoid running forward as much as he is asked now and then failing to track back at times).
The last piece in the puzzle, an important one though, is our goalkeeper. I don't think we need any signings there. Probably start introducing Henderson gradually to the first team. I love De Gea for all he's done for us, but his best days as past him. He's become too inconsistent. Henderson has shown that he is a very good goalkeeper playing for Sheffield. So it's not like we don't know what we have. His loan spell there made us realise that we have a good talented young goalkeeper at our disposal. I wouldn't give the blunder against Sheffield too much attention, he needs a proper run of games to get in form and reach his potential.
With that said, our defence has not been fixed yet. No.