Simple fact of the matter is 80% of the franchise/elite level QB success stories that ended up playing as rookies showed you SOMETHING right away, even as rookies, to suggest that they were really good.
Josh Rosen had a really bad year. His offensive line doesn't explain why he had the 2nd worst passer rating in the NFL even on plays where he was kept clean of pressure. And don't give me any crap about Larry Fitzgerald, Christian Kirk, David Johnson, Ricky Seals-Jones, and Jermaine Gresham being so awful that the team could have the worst offense in football despite their QB being secretly pretty good. They're not that bad. Hell, they're not even bad at all. That offense had a QB problem.
Josh Rosen had a 26.6 QBR for the 2018 season. Do you realize how bad that is? We haven't even seen a QBR that low among qualifying starters in the NFL since Blake Bortles was a rookie in 2014.
I liked Josh Rosen well enough coming out of the draft. I like him just like I have liked many guys in the 1st round over the decades, as a dice roll that you're figuring has perhaps a 40% chance of panning out to be the kind of player you want. That's what you're looking at as a guy like that.
Except that like I said, about 80% of the guys that do pan out, if they played as rookies, they looked pretty good.
- Peyton Manning was 5th in the NFL in passing TDs as a rookie, which back in 1998 the way the NFL was back then, was a BFD.
- Matt Ryan finished his rookie year Top 5 in QBR, with an 88 passer rating, improving a 4-12 team to 11-5, and a 29th ranked offense to Top 10.
- Cam Newton took his offense from 32nd in the NFL to 5th in the NFL in points scored overnight.
- Andrew Luck was Top 10 in QBR as a rookie and led his team from 2-14 to 11-5 overnight.
- Russell Wilson was Top 3 and had a 100 passer rating that same rookie year.
- DeShaun Watson had a crazy 83.6 QBR as a rookie, starting 6 games and putting together a 103 passer rating.
- Ben Roethlisberger was literally undefeated as a rookie starter, assembling a 13-0 record with a 98 passer rating.
- Baker Mayfield finished the year with a 94 passer rating, taking an 0-16 team and going 7-7 during his time as the QB.
Really if you look at it, the only guys that would have been considered fully successful top QB picks, that played as rookies, and sucked as rookies...are Eli Manning and Carson Wentz.
You could maybe count Jared Goff as among the success stories that started, and played poorly as rookies. You could maybe count Donovan McNabb. You could perhaps count Alex Smith, Joe Flacco, Matthew Stafford, Ryan Tannehill, Marcus Mariota, or Jameis Winston. Maybe you'd count Derek Carr, or Andy Dalton.
Personally, I would think we'd have higher standards than that. And I'd also note that most of those guys, even if they did have poor rookie years, had better ones than Josh Rosen did.
The point is simple Bayesian updating says you have to look at your previous evaluation and update it with the new information. If you thought the guy had a 40 or even 50% chance of panning out before, and 80% of the guys that do pan out and were asked to play as rookies actually showed you something positive as rookies but he did not, then you're looking at maybe about a 10% chance that he's still the guy you were looking for him to be.