Johnson isn't the only greenhorn impressing. Here is Around The NFL's latest Rookie of the Year watch:
1. Marcus Mariota, Tennessee Titans quarterback: Why it's called "passer rating" instead of "quarterback rating": Mariota took seven sacks and fumbled three times, yet still generated a rating of 96.3 against the Cleveland Browns. He's leading the league with a 129.9 rating -- just ahead of Aaron Rodgers (128.4), Carson Palmer (124.4) and Ben Roethlisberger (122.6) -- entering Week 3.
5. Matt Jones, Washington Redskins running back: There are only a handful of human beings who can move like Jones at 6-foot-2 and 230 pounds. Jones led the Redskins' new ground 'n' pound attack in Week 2, blowing through the Rams' talent-laden front seven for 123 yards and a pair of touchdowns. Against the Giants on Thursday, however, his superlative abilities yielded only pedestrian results. He was held to 38 yards on 11 carries and cost the Redskins a touchdown when he fumbled the ball out of bounds in the end zone for a touchback.
7. Amari Cooper, Oakland Raiders wide receiver: Renowned for his savvy route running, Cooper impressed with his speed in racking up 109 yards and a touchdown versus the Ravens. Per Next Gen Stats, Cooper reached 21.38 mph on his 68-yard touchdown reception, the fourth-fastest time by a wide receiver in Week 2.
8. David Johnson, Arizona Cardinals running back: A high-end NFL athlete, Johnson has recorded the fastest time by a running back this season and launched a 41.5-inch vertical leap at the NFL Scouting Combine. He's the first player in NFL history to score a touchdown via run, pass and kick return in his first two games.