I agree he made an error of judgement in that first half. I suspect the thought Boro might have been weakened because of their absentees, and went for an attacking line-up. Unfortunately, several of our most consistent players spent the half giving the ball away, so mounting attacks was impossible.
And what happened to Evans? That was the worst game he has ever had. I'm sure Smallwood would have got a foot in more, but that's being wise after the event.
Slightly off-topic on the matter of the centre-backs, I wonder why the concept of the "sweeper" (libero) went so dramatically out of fashion? I actually played in that role myself for a while, but switched to "stopper" midfielder. I suppose that's the same job, but IN FRONT OF the back four!