Rooney isn't that creative for a 10 in the sense he is predictable at times therefore I can see Louis van Gaal using him as a 9 and why not? He will guarantee goals in my opinion and he is very creative for a 9.
Herrera and Mata both could be excellent 10's and I see the manager using them for this, there are plenty of games, rotation will be necessary next season providing we get European football and when you factor injuries etc, what a wonderful position to have both Herrera and Mata compete for the 10 position.
But it goes further, these two players are different, it allows for more tactical flexibility and so if both players are in good form the one selected could be based on the opposition strengths and weaknesses.