Well Chelsea is already title contenders. You sell Rooney to them and you hand them the league and simultaneously doom Moyes' first season.
You could have said that Torres guaranteed Chelsea the title a few years ago when Ancelotti signed him in January.
Chelsea have a lot of attacking players, it's already a bit crowded up top with Lukaku, Ba and Torres all expecting to play and they have Lampard, Mata, and Oscar who all would prefer to be playing in behind the striker. Rooney is also a big personality and he would be walking into a locker room full of big personalities.
I'm not saying that I don't think Rooney would be successful at Chelsea, I do think he would, just that you never know.