Pienaar and Baines was a makeshift combination that worked well.
It's not just about those two, as they play triangles with another player. There is nothing to suggest Mirallas couldn't do the same, or anybody else. What we do know, is that if Pienaars natural game is right sided, and Mirallas' natural game is left sided, and Fellaini's natural game is defensive midfield, and so on, then everybody is playing out of position to cater for one aspect.
I expect the sensible thing to do would be to experiment, knowing we can always revert if necessary.
If it was proved that Oviedo for example, can play left back, and also attack down the line, then it gives us the option of selling Baines and adding 2 or 3 much needed players. "Shock horror " some will say at the thought of selling Baines, but it 'is' a plausible option.