I don't think losing Clemons hurt us very much, Irvin and Scofield have played well and the bar to clear for 2013 Clemons is a low one.
Though I think McDonald very likely had a career year last season, his sizable contribution to the 2013 pass rush is basically what's missing from the 2014 version. Williams has played very well for the most part, but he's not having the pass rush season McDonald had last year.
I think our pass rush has suffered from a few things beyond personnel. The main one is that 4/5 of the QBs Seattle have faced this season are very good or elite QBs. The only one of the bunch that is easy to sack, Rodgers, was not holding the ball as he customarily does. Really, the only game with a great chance to rack up sacks was the road game in Washington. I'm not saying Seattle has been secretly great, but even if they were, you'd hardly know it from the way this season opened opponent wise.
The lack of press has made offenses go more dink and dunk than ever, and it's harder to sack a dink and dunk QB. And let's be real, our secondary performance so far isn't a shadow of what it was last season. QBs are frequently finding wide open WRs in 3 seconds or less. That wasn't happening last year, and probably explains the drop in pressure rate (which doesn't care how long a QB holds the ball) almost by itself.
It's still fairly uncommon for a QB to have 5 seconds to throw against Seattle unless he has elite pass protection and/or Seattle sells out that play to jam the run gaps.
The next several games will give Seattle some good opportunities to pad their pass rush numbers. If they are still struggling a month from now, I'll be concerned then.