I'm pretty sure that when he (Rooney) played as a proper striker (not a number 10 or a midfielder like he has been lumped with as of late) scored 34 goals in 44 appearances (09/10) and 34 goals in 43 appearances (11/12).
He'll score more than Giroud (easily) this season and is perceived to be the weakest out of Aguero/Costa/Rooney which is worrying. If we want to challenge we need a big time striker.