Being slightly leaner, may increase performance, but it also may not. Getting "lean" - that is having low body fat, is done by being in a calorific deficit. Training to be in a deficit and training for performance are two different things, hence why bodybuilders at low BF percentages are relatively weak just before a show. As such being in a calorific deficit doesn't aid performance, in fact, it likely hinders it.
Some people are naturally leaner than others, some have to work harder for it. Maybe Rooney could work a bit harder, but to be honest, in my opinion his drop in performances is as likely to be down to the sheer number of games played catching up with him and the fact that he isn't probably a "natural" athlete.