Mack entered last year as a popular sleeper, but after only 18 offensive snaps through Week 5 due to lower-body injuries, it looked like the talented but brittle back was staring at a lost season. He got healthy Week 6, though, and was immediately off to the races, finishing with more than 1,000 scrimmage yards despite missing four games. The South Florida product is extraordinarily elusive in the open field, but his secret weapon was the rise of the Colts offensive line. The addition of Quenton Nelson pushed the unit to elite status, so Mack found himself with bigger holes and a lot more running room than he had the year before. He's not an especially physical runner (his 16 broken tackles were well below average for a feature back) and he had less than half of the team's carries inside the 5-yard line, though he converted six of 13. Mack still needs to improve as a receiver and show he can stay on the field to become a true bell cow, but with only Jordan Wilkins and Nyheim Hines competing for carries, Mack is set up for a productive 2019.

Few rookie campaigns have been as predictable as Mack's 2017. Stuck behind Frank Gore on the depth chart while playing in an offense that didn't present much of a vertical threat without Andrew Luck under center, the fourth-round pick out of South Florida couldn't produce big numbers, but he showed flashes of his big-play ability with seven runs of 15 yards or more on only 93 carries. At 6-0, 210, Mack compensates for his thin frame with speed that appears better than his 4.5 40 time, along with ankle-breaking moves that routinely leave defenders grasping at air. It's also worth noting he played through a torn labrum in his shoulder most of 2017 - ultimately requiring offseason surgery - which could explain his reluctance to run through tackles or fully engage blitzing pass rushers. Returning veteran Robert Turbin figures to be limited to short-yardage work once he returns from a four-game suspension, and while the Colts drafted Nyheim Hines and Jordan Wilkins to compete for snaps, the former profiles as a pass-catching specialist and the latter looks like a lesser version of Mack. If Luck can return and force safeties out of the box, the second-year back has the talent to exploit the extra space. Mack faces another obstacle after straining a hamstring in the first week of the preseason, leaving his Week 1 availability in doubt.

A fourth-round pick out of South Florida, Mack potentially gives the Colts a big-play hare in their backfield to complement Frank Gore's slow but steady tortoise act. Mack showed incredible elusiveness and balance at USF, routinely making wouldbe tacklers miss en route to six TD runs of 40 yards or more in his final college season -- and he's got the burst to hit a hole with power when he identifies one. But he too often looks to bounce a play outside for a home run rather than settling for a smaller opening inside, something that could get him in trouble against faster NFL defenders. Gore's tutelage could help the rookie temper those instincts and learn when it's best to take what he's given. Mack likely will fill a change-of-pace role in 2017 but could be in line for bigger things once Gore retires.