Its very much the tactics.
We've played extra players at CB - Ander, Fellaini, McTominay and only Fellaini has worked out and that was against specific teams who are going to put lots of balls into the air for him to help Smalling to clear. Teams that keep the ball on the floor and look to break or play clever passes around the box inviting fouls, Fellaini is much less likely to work out.
So we've done that and it hasnt actually helped us defensively - other than Fellaini against specific teams. Ander played his man onside, we conceeded. McTominay with a whole host of problems. Its just been too many cooks in the kitchen and confused what people's responsibilities are. And obviously having the extra player hasnt helped us in attack.
We went down that path after Mourinho destabalized our defense, going into the season without our rock at the back Smalling nor his partner for our good run last season Jones. He went with Bailly and Lindelof, they were poor and conceeded so Mourinho went this ultra defensive route that didnt help us defend and its spiraled out of control from there.
Go back to playing a normal 2 CBs, or 3 CBs who know what they are doing and the players will know what their responsibilities are, and if their man scores they know they are to blame. They can take some responsibility. But playing Ander at CB? Well he's not a CB of course he'll make some mistakes adjusting. McTominay? He's not a CB, of course he'll make mistakes adjusting. And yes, at some point Fellaini will make a mistake too and it'll be the same, just a symptom of playing him in that position.