Both Robertson and Tierney play as part of a back four for their clubs. In fact, I would guess that’s the case for most of our defenders. Playing with three centrebacks and wingbacks takes time to adapt to and practice. It’s maybe something to try out in some friendlies but not just now.
More importantly, we struggle to pick two central defenders as it is, without adding a third, and with Calum Paterson out injured we don’t have any options for the right wingback. Tierney covered rightback against Slovenia and did well (though they barely tested him all game) but a right wingback has to provide your width in that system which you can’t do playing someone on the wrong side of the pitch.
The problem isn’t that 4-2-3-1 is not the correct formation for us, it usually is. The problem is that since he took over, Gordon Strachan has played it relentlessly regardless of our opposition.