Lol I see you've never played football.
You do realize that defensive linemen have assignments on every play depending on down, distance, and tume remaining. Its not just "Hey Greg get after the QB"
He's only 24 years old, who again to anyone that knows football has played at a very high level to this point.
But you just look at sack stats and write him off because he didn't have 10+ sacks this season.
I'll bet you barring injury, Rousseau gets over 10+ sacks 3-4 times the remainder of his career.