I'm not trying to say that he was bad or good. He was effective behind the two strikers, and it allowed him the freedom to move back and forth between midfield and the final third. Plus, it allowed him to be the least disciplined in his game and allow him to track back and support the team off the ball. However, I'm commenting more on his style of play than his performance quality.
He takes up very good positions behind the strikers, but he doesn't stretch teams with horizontal movement, something that's fundamental for attacking midfielders. Also, he needs to demonstrate the close control that can prevent us from slowing down our passing tempo and rhythm. Several times, I've noticed some of his bad touches slow us down quite a bit when we were on a good rhythm and tempo. Finally, he struggles in tight spaces, something attacking midfielders should be comfortable with. However, he's excellent at finding himself in open space, and that's something I would expect from false 9's/second strikres. Also, his passing and vision are good enough to link up with others and put through runners on goal or dangerous areas, but he's not someone who can dictate our tempo and passing rhythm. Finally, his finishing is an asset that cannot be suppressed in any way, and as a second striker/false 9, he can ghost into goalscoring areas or take up goalscoring positions.
It's similar to how I see Muller. He's played behind Gomez and Mandzukic many times for Bayern, but has this ever meant that he was an attacking midfielder? No, he was always playing like a second striker, but what makes him different from typical second strikers is that he is comfortable out wide unlike many second strikers nowadays.
Anyways, back to Rooney. Rooney does well behind the two forwards because his energy levels mean that he cannot be easily marked out of the game like Mata can, and we can play strikers who are clinical. Plus, having other great attacking players like di Maria and Herrera means that Rooney will only have to deal with 1 opposing marker most of the time. However, I believe we can be a better team if we have a more energetic, dynamic forward playing off of Falcao than van Persie. Rooney can play as he usually does right now, but by playing off of Falcao, we can be an even better team IMO rather than playing behind van Persie and Falcao.