That's not true at all. The defense going south wasn't coupled with Bush's injury.That was coupled with Bush's injury and you would have to hope that Highsmith will make a second year jump. No question sack production went down. You have to hope for three things.
Bush got injured in week 5. Between week 5 and the end of week 11 (when Bud got hurt), the Steelers went 6-0 and gave up an average of 15.7 points per game. After Dupree went out, they were 1-4 and gave up 24.8 points per game. Then they got trounced by the Browns in the playoffs, with the defense allowing 41 points.
Seems pretty clear to me that it had a lot more to do with Bud than Bush.