Shelvey is the more talented player plus plays slightly further forward which would allow Ross to drop back if McCarthy is sold. The more I think about it, it's not that stupid an idea plus would probably make us a net 15m allowing us to buy a quality winger or even CB/GK.
The problem with both Besic, Barry and McCarthy is there's no goals in them and teams have learnt that all they have to do is stop our fullbacks. Shelvey and Ross in a midfield 3 gives us a potent attack and Besic works best as the furthest back in a midfield 3.