Rossi never scored 20 goals or made 10 assists in La Liga. It's too optimistic to expect him to do the same in a league with less goals and tougher defenses, against teams who mostly play defense when they face Juve, unlike the Spanish teams playing vs Villarreal.
So far he's a man who scored 11, 12, 10 and 18 goals as well as assisting 3,7,3,5 goals in the same seasons. A good UEFA cup campaign in 2010/2011 does not make him a great scorer.
Maybe one day he will become a +20 goal striker but until that day comes I give him the same chances to achieve that as I give to Quagliarella.