I have heard different reports about whether Sage plays with a back 3, 4, or 5, or varies it up from game to game. I am a big believer in the strengths of the players dictating the system, not the other way around. When it comes to our full backs, I have always felt that Mitchell better suits an old-school back 4, but that Munoz better suits the wing-back system.
That creates a bit of a quandary. I would have said that, as modest as his attacking abilities are, Mitchell is probably a better wing-back than Munoz is an old-school full back. On that basis, probably best to stick with a back 3/5.