Surely this would see Ross moved back to a traditional central midfield role. With a whole summer of training, he could learn the basics to develop in that role, for example, tracking back, closing down, interceptions, being strong in the tackle. None of which should be above him with hard work and application, as well as top coaching.
Koeman plays 4-3-3 as much as he plays 4-2-3-1 mate.
A player of Mata's quality wouldn't be wasted anywhere in our team (either behind the forward or on a wing), as everything would go through him.
At United he was classed as a luxury player by LVG, so when he was out on the wing he tended to get isolated. Wouldn't be the case here.