I suppose that comes down to the job brief on the other side of the ball. Rooney put in a lot of defensive graft last night right until the end of the game, and whilst his attacking play was not becoming of a #10, he really helped out whilst we were under the cosh. None of the players you mentioned can do that kind of defensive shift, either by design (Mata, weak, low stamina etc.; Pereira inexperienced and out of his depth) or just because they haven't the ability to be switched on defensively frequently enough, which I think applies to both Herrera and Fellaini.
In the attacking sense, Mata is looking the part lately, so could have the #10, but there's no way he gets the blocks on that Rooney did, and LVG wouldn't like that. Herrara seems to be earmarked to come on for Mata or a deeper midfielder, so there's no overlap with Rooney there, either.
If LVG really wanted to pressure Rooney for his position, he would've brought a #10 in over the summer who matches his requirements for the role. I honestly think what we have here isn't to LVG's liking for the position with or without Rooney, so he'd rather put his eggs in the basket of Rooney getting better than giving someone else the role.