I completely agree. Although for me it isn't the 'Di Maria problem', it's simply the 'playing 3 strikers at once' problem...
We've seen that Mata has lacked 'impact', and I agree that his contribution over the last 3 games hasn't been massive, but one thing he does that seems to go unnoticed is
holds the position of a number 10. He can be underwhelming, but he at last fills the position he's played in.
Rooney on the other hand has become almost synonymous with 'doing more than is needed' in whatever position he plays - a CF via a box to box midfielder - he's everywhere. Much to most fans delight. But he's playing the number 10 the shape of the team suffers due to Rooney's business and lack of positional discipline.
Ultimately,
Rooney is a striker, and the number 10 role in modern football has become a specialist position - there aren't many top, top teams who just take a striker in his late 20s and plonk him in that role.
To make it worse, LvG has openly stated that Rooney, as his captain will start pretty much every game. So it seems this is set to continue.
And then on top of that you have the 'playing two number 9s together' problem that has undoubtedly cost us points already. RvP and Falcao are players that should each be leading the line. They confuse and complicate each other's game when played together and perhaps most importantly - playing them both deprives us of an extra body in midfield (thus as
@Pogue Mahone said mediocre teams have walked through us).
I'm fine with LvG putting Rooney for in Mata - he's made Rooney his captain and now has to deal with the consequences of that. But regarding the two number 9s, Van Gaal has to pick one over the other. Otherwise we're almost playing with 10 men.