I interpret the question as meaning which guy in his rookie season was the most impactful for that particular season.\
Logically, I think we would look for that player that was a rookie and made the difference between going to to the Super Bowl or perhaps not going. If such a scenario existed, how could any rookie be more impactful?
The Cowboys shocked the entire NFL in 1975 when they went to the Super Bowl. That was considered a rebuilding year. The Cowboys went 8-6 in 1974 and all the original defensive stars were retiring as well as the offensive line and receivers.
The RB squads had been depleted by the new WFL. They went from having Calvin Hill, Walt Garrison, and Duane Thomas to a 2nd yar back, Robert Newhouse, and a veteran that had been cut by his original team, Preston Pearson.
These two RB's were adequate and that wasn't good enough to keep the Cowboys in the playoffs in 1976. The Cowboys were always a dynamic running team. Average wasn't going to be good satisfactory. The running game needed a good motor.
The Cowboys chose a jet engine. They made a trade with the Seahawks for several players and some other draft picks and moved up to the #2 spot to select Tony Dorsett.
That was the difference in 1977. The defense was becoming elite again with "Doomsday II" but they needed someone to open things up and Dorsett was perfect. If you never saw him play, he was the kind of RB that makes fans of both teams hold their breath.
As a rookie, Dorsett had a thousand yards in a 14 game schedule. He only started 4 games.
In Dorsett's 2nd NFL game he ran 7 times for 62 yards. In his 3rd game he ran 10 times for 72 yards. in his 4th game he ran 14 times for 141 yards.
In Dorsett's 2nd official NFL start, he ran 23 times for 206 yards. He scored the first TD of the game on a 1 yard run and he scored the last TD of the game with an 84 yard run.
In the playoffs the rookie ran 51 times for 220 tards and 4 TD's.
In 1977, Tony's rookie season, the Cowboys went from being an average running team with an average offense to the #1 offense in the league in yardage.
Tony Dorset's won a college national championship his senior year and then a Super Bowl his rookie year in the NFL.
Can anyone name another player whose impact was responsible for winning the Super Bowl during his rookie season? Is there a better example of how to impact the team in a positive way?