What were you watching in regards to Spillane?  I thought the complete opposite -- this was one of his best games as a Steeler, leading the team with 7 tackles, 5 solos, 1 TFL, and the game-clinching stop of Dalton on the 4th down sneak attempt.  Plus he was decent in coverage.  No idea what you're talking about.
And I disagree with the other posters who said that Pickett was under constant pressure the whole game, and the Steelers never having anybody open.  Yeah, KP was pressured a lot today, but he also had plenty of time on a number of occasions, and missed several throws, including a wide-open Pickens on the very first play from scrimmage.  Rookie or veteran, there's no excuse for those.
Here's what I've noticed about the two "superstars" on the Steelers defense:  While Minkah is replaceable, Watt is not.  Along with his own prowess, Watt coming back takes double-teams away from Highsmith.  Highsmith gets more pressure because he's no longer being doubled.  That pressure from both sides makes the whole defense better. When Fitzpatrick misses a game, other secondary guys step up.  The drop-off in talent/production is negligible in the secondary.  The drop-off at outside linebacker is like falling off a cliff.
Good to finally see some production out of the running game, but this offense is still hard to watch most of the time.