Looking at recently drafted quarterbacks, it’s hard to prove any consistent drop in play among second-year players. There were minor declines in some:
Year One-60% 4051 yards 21 TDs 17 INTs 6-10 record.
Year Two-58% 3869 yards 19 TDs 12 INTs 7-9 record.
Year One-53% 2267 yards 13 TDs 20 INTs 2-8 record.
Year Two-59% 535 yards 6 TDs 1 INTs 1-2 record (injury)
Year One-61% 3440 yards 16 TDs 11 INTs 11-5 record.
Year Two-58% 2916 yards 22 TDs 14 INTS 9-5 record
Two of those players had negligible drops in completion percentages, tossed more interceptions, or just missed time from injury, but it’s hard to say they suffered a major slump in season two.
There are just as many recent examples of players who improved their play in their sophomore seasons, some in small increments, some in larger amounts:
Year One-60% 2971 yards 14TDs 12INTs 11-5 record.
Year Two-63% 3613 yards 21TDs 12INTs 9-7 record
Year One-54% 1855 yards 10 TDs 18 INTs 3-6 record.
Year Two-61% 3451 yards 25 TDs 6 INTS 10-6 record.
Year One-59% 1001 yards 9 TDs 5 INTS 2-3 record.
Year Two-64% 3497 yards 20 TDs 14 INTs 7-9 record.
The players there improved stats, even if they didn’t improve their team’s record, something that isn’t always relevant when discussing individual positions.
This repeats itself going back through the history of the league.
Peyton Manning, Tom Brady, Dan Marino, and Eli Manning all improved from year one to year two. Ben Roethlisberger, Drew Brees, and Brett Favre all had slight dips in their play. There seems to be no discernible pattern to predict how well a quarterback will do after their first year.