I'm inclined to agree.
Whilst Mitchell may be getting up the pitch so would Paula Radcliffe or Mo Farah, and they'd probably be as about effective as him. Great player, one of my favourites of our current lot, but an attacking wingback he is not. Munoz has been a different player for a while now too.
The main difficulty for me would be the transition to a back four having played our formation for a while now. That could take a number of games on it's own. I don't buy into the notion that professional centre backs will be all at sea in a '4 at the back' set up though, they would have played there and got to this level having done that for the majority of their football playing lives.