It feels like this is a similar situation to the one Mata faced at Chelsea and it hasn't surprised me at all that Rooney has started the last two games for us. While Rooney isn't a better #10, his understanding is significantly better as he's more positionally disciplined. Didn't Van Gaal say something about Mata not impressing him in the role?
Mata is a very good player, but when it comes to systems like ours in which he has to press high and be expected to play as the link man in the middle, he'll come up short. I believe the following reasons are why Van Gaal has opted for Rooney in the past two games:
1) Mata struggles physically. Doesn't press well and does have the tendency of being "lazy", but this is a minor point as we're not challenging for the league.
2) Doesn't play like a #10 should. Doesn't possess the control and skill Silva and Özil have which allows them to play in tight spaces. Often finds himself moving laterally to try and find space but that results to blocking passing lanes. That's why you'll notice him
3) Poor under pressure - lacking the agility to turn - and doesn't shield the ball well hence he moves away from the middle to find space. Against Leicester he was pressured and did what he usually does in such situations - he panicked and didn't protect the ball.
Chelsea replaced Mata with Oscar and immediately they functioned better as a team, and not because Oscar is a better #10 but because he excelled in key areas of Mourinho's system. Rooney has come in and done better in the role, and again that isn't because he's a better #10.