No defence (back four or back three) can operate successfully, when midfield and front players continuously lose the ball in the wrong areas. We have seen time again when our defensive set-up gets really stretched is when a quick turn-over to the opposition happens. Pogba and Fred are the two most often midfield culprits and Rashford and Martial up front, at times head down they run up blind alleys and lose the ball, allowing the opposition a quick turnover. Bruno, McTominay and Greenwood are not exempt from this criticism either. In fairness those guilty of such play do try to recover the situation, but there is no obvious linkup with the back four, there is no 'fall back / break out containment' procedure, at least none apparent and in the end players trying to recover the position, simply by chasing the player they lost the ball to and often make it worse by creating defensive overlaps in the wrong places, allowing attackers free space elsewhere.
Yes, I agree we could do with a different option at CB, Maguire and Lindelof are too similar in style, but that wont stop gaps appearing in front of them due to the above, only when Matic plays do you sense any real chance of recovery, but he is getting long in the tooth and if he needs to be 'at it ' from the first whistle he will not last a full match. We now have four decent full backs two able to defend AWB/Williams, effectively, two who carry a threat going forward Shaw /Telles and Tuanzebe and Bailly are coming into the reckoning again. Obviously our midfield and forward players need to take risks and they will lose the ball, at times in the 'wrong place', but there needs to be a fall back, containment plan, starts in the oppositions third, overlaps in the mid third and bolsters the final third in front of our goal. As with the military situation ability to retreat 'in formation'/'in order' is not giving up, many retreats have been the basis on which victories are won. This does not mean 'parking the bus' in football terms, but rather responding with on-field management and tactics determined for given situations. 'Dead ball' situations at corners and free kicks is practiced why not some classic cases of recovering turn over/lost ball situations as well?