Plenty of guys are even given a few weeks to wait and still prove to be not at all ready.
Quarterbacks since 2003, drafted #52 or higher, who started 9+ games as a rookie:
Week 1 starters in italics
Winners (10)
- Jameis Winston - probably?
- Marcus Mariota - probably?
- Derek Carr
- Andrew Luck
- Andy Dalton - Meh
- Cam Newton
- Matt Stafford - 3rd rate starter
- Matt Ryan
- Joe Flacco -3rd rate starter
- Ben Roethlisberger
Questionable (4)
- Carson Wentz - jury out
- Teddy Bridgewater - Injured
- Ryan Tannehill - mediocre at best?
- Sam Bradford - Borderline starter
Losers (12)
- Blake Bortles - fraud
- Geno Smith - backup material
- EJ Manuel - backup material
- Robert Griffin III - Unemployed
- Brandon Weeden - 3rd string garbage
- Blaine Gabbert - nope
- Christian Ponder - nope
- Jimmy Claussen - gone
- Josh Freeman - out of the league
- Mark Sanchez - Garbage
- Vince Young - Out of the league
- Matt Leinart - Out of the league
Quarterbacks Drafted Since 2003 Who Were Held Back as Rookies:
Winners (3)
- Aaron Rodgers (0 rookie starts)
- Eli Manning (7 rookie starts)
- Philip Rivers (0 rookie starts)
Questionable (3)
- Alex Smith (7 rookie starts, 136 career starts)
- Jay Cutler (6 rookie starts, 139 career starts)
- Jared Goff (7 rookie starts)
- Paxton Lynch (2 rookie starts)
Losers (11)
- Johnny Manziel (2 rookie starts, 8 career starts, out of the league)
- Jake Locker (0 rookie starts, 23 career starts, retired due to injuries)
- Colin Kaepernick (0 rookie starts, 58 career starts, it's complicated)
- Tim Tebow (3 rookie starts, 16 career starts, out of the league)
- JaMarcus Russell (1 rookie start, 25 career starts, out of the league)
- Brady Quinn (0 rookie starts, 20 career starts, out of the league)
- Kevin Kolb (0 rookie starts, 21 career starts)
- Drew Stanton (0 rookie starts, 14 career starts)
- Kellen Clemens (0 rookie starts, 21 career starts)
- Jason Campbell (7 rookie starts, 79 career starts)
- JP Losman (0 rookie starts, 33 career starts)
Guys Who Are Still Being Held Back
- Christian Hackenburg (has been unable to beat out Ryan Fitzpatrick, Geno Smith, and Bryce Petty)
Guys Who Never Made It (<8 career starts)
What we see here is that since 2006 there isn't been a single successful quarterback who was held back significantly during his rookie year. You have to go back over 10 years to find an instance of this turning out well for a high pick. This strongly suggests that quarterbacks who aren't deemed ready as rookies are highly unlikely to ever break through. I'll note that the 3 who DID achieve this all did so behind Hall of Fame level quarterbacks (Favre, Brees, Warner), and the vast majority of the failures had no such high level quarterback.
There's also this: Even among quarterbacks who start a significant number of games as a rookie, there are more failures than successes. We will never know if starting as rookies "before they were ready" stopped them from realizing their potential.
Other notes: DeShone Kizer is only 21. The only 2nd rounders to start in Week 1 among the players I looked at were Andy Dalton and Derek Carr who were both 23 years old when drafted. Winston and Mariota were 21 and have found early success as immediate starters, but they were #1 and #2 overall picks. There haven't been many 21 year old quarterbacks. Rodgers and Alex Smith were 21 year olds who were not thrust immediately into the starting job and ended up alright. Gabbert and Freeman were made starters as rookies, and it turned out badly for them. Jared Goff will be an interesting case.
I sincerely hope to see Kizer by Week 5, ideally not earlier than week 3.