I don't think that we should forget that some of our best recent form has been with 3 at the back. I'm not convinced any of our centre halves (maybe Williams aside, if he's with the right partner) is good enough to play in a two, whereas the three provides extra coverage and compliments the likes of Holgate and Funes Mori more. Also we gained control of the Bournemouth game by switching to three at the back, and away to Boro, I think he's more likely to go with the back three.
Yeah, I definitely think this is the case right now, until we hopefully upgrade our CB's in the summer.
However, we did have a firm grip on the Bournemouth game with the xmas tree, until we started shitting ourselves, and yes the three at the back switch came at just the right time.
I think maybe start the same (4-3-2-1) and be prepared to switch to three cb's if the going gets tough.