By QB rating I assume you meant his passer rating? I'm not looking for Tom Brady or Chad Pennington or even J.P. Losman when I watch Cassel. What I saw in the first series was a seventh round draft pick who has taken very few game reps in his college/NFL career and is working with a mix of 1's, 2's, & 3's against a similar mix in the first game of the preseason. On that first series he looked reasonably decisive considering he and his #1 WR were working off different paragraphs if not different pages. Rough yeh, but not shaky.
On the first 3 and out the Ravens penetrated once to disrupt a Maroney run, rushed 6 and tripped (inadvertently) the open WR (who from some views may have slowed for the catch, that's for the coaches to know) causing an incompletion, and made a nice play on a bubble screen that never worked once last season when run by that WR. I saw things Cassel could have done better, but I didn't see a guaranteed 8-8 season if Brady went down - not when the game plan is altered to take advantage of Cassel's strengths.
As I've said elsewhere here, all three QBs looked solid for where they are in their development - lot's of room to get better, but in a team sport they'll have some help. Gut looked okay, but okay working against lesser competition, with lesser talent around him. Note that Gut was one-hopping some of his throws, I'm not ready to say it's arm strength, more likely it's knowing when to bring the heat and when to throw with touch. Gut was thinking his way through where Cassel was reacting from training - that puts Cassel ahead of Gut on my scorecard.
O'Connell looked very promising, and very limited. He was turned loose to just go out there and play without really trying to execute the decision trees the other QBs were using - it looked fun against the Ravens' scrubs (and recall Gut and O'Con both had some veteran OL mixed into the O-line - just look at the line surge on the TD drive, the Pats FA vet signings were out-classing those Ravens' scrub DL).
I'm going to watch some more, but I didn't see anything in that game to change the depth chart behind Tommy.
Having watched the game through once, I'm psyched about this team. The younger players all showed why Pioli went looking for them. Chad is the only one whom I still have questions about, but I'm not ready to write him off either - maybe after another series.