His stats in terms of goals/assists per 90 were actually among the best of his career last season:
http://www.statsbomb.com/2014/01/statsbomb-mythbusting-five-years-of-wayne-rooney/
He very much did play second fiddle to RvP, but still popped up with some important goals. It's often overlooked that in the period between December '12 and March '13, he scored 13 goals in 16 games, not all of them starts.
Those goals included a winner against West Ham, the only two goals against Southampton, two at the Etihad, a winner against Fulham, and a winner against Reading. A very decent contribution, in other words.
His season started badly and ended badly (mainly because he wasn't playing), but he was never as bad as what's been made out.
But for a lot of fans, RvP was the shiny new toy in 2012/13, and so the perfectly good toy they already owned was obviously going to be cast aside and neglected for a bit.