Rookies are the ultimate unknown in the NFL. Teams review hours and hours of game film, analyze enough numbers to make most banks jealous, and scout players at individual workouts, but there's still an unknown quality to rookies. It is difficult to predict how they'll perform as professionals.

When they do perform well, general managers and coaches look like geniuses. In Year Two, the focus turns to avoiding the dreaded sophomore slump. Fantasy owners must be keenly aware of this -- rookie success doesn't always guarantee sophomore stardom.

With this in mind, we've highlighted a handful of standout 2016 rookies who could take a step back in 2017. Note: 2017 average draft positions from Fantasy Football Calculator are in parentheses.

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Sophomore Year Outlooks

Final stats: 1,313 rushing yds, 6 TDs; 29 rec, 298 receiving yds, 1 TD

Howard was one of the great waiver wire gems of the 2016 fantasy season, a fifth-round pick out of Indiana who climbed from a third-stringer on his own team to the second-leading rusher in the NFL. He undoubtedly helped countless fantasy owners win their league titles, and he will enter the 2017 season as one of the darlings at the running back position.

However, before you invest a first-round pick in Howard, it'd be wise to revisit the case of Todd Gurley. You remember him, right? He wasn't a late-round pick like Howard, but he did explode onto the fantasy scene with multiple 100-yard games, particularly in the second half of the 2015 season. He entered the 2016 season with considerable hype, but failed to even reach 900 rushing yards.

Gurley was the focal point of an offense without a proven quarterback or dangerous options at wide receiver. Chicago could very well part with Jay Cutler and Alshon Jeffery this offseason, and their replacements could be unproven. That leaves Howard as the only reliable source of production, a fact that won't be lost on opposing defenses. Howard has tremendous upside, but his team and more specifically his quarterback could hold him back in 2017.

Michael Thomas, WR, New Orleans Saints (2.09)

Final stats: 92 rec, 1,137 receiving yds, 9 TDs

The Saints selected Thomas in the second round of last year's draft, and he made an immediate impact in one of the NFL's most potent offenses. The former Ohio State standout finished first on the team in receptions and touchdowns, and second in receiving yards behind Brandin Cooks (1,173).

Thomas led all rookies in receptions, yards and touchdowns. He was nothing short of brilliant, but he could be slightly less brilliant in Year Two. Why? Well, like Howard, opposing defenses will have a full year of tape to scout, so if Thomas doesn't adjust properly, he could struggle.

With Cooks, Willie Snead, Coby Fleener, Mark Ingram and even Travaris Cadet in the mix, that's plenty of competition for targets from future Hall of Famer Drew Brees. Thomas still has considerable upside, but he's currently being drafted as the ninth wide receiver, on average. That's a bit high for a wide receiver with only one season of pro production.

Tyreek Hill, WR, Kansas City Chiefs (3.09)

Final stats: 61 rec, 593 receiving yds; 267 rushing yds; 12 total TDs

Rookie or not, Hill turned in one of the most dynamic seasons in recent memory. The speedy wide receiver/returned gashed teams through the air, on the ground and in the return game, even becoming the first player since Gale Sayers with a rushing touchdown, receiving touchdown and return touchdown in the same game.

The obvious concern with Hill is whether or not a pace like that is sustainable in Year Two. There's no reason to believe he'll become less involved in the passing game, so he should be able to top his 61 receptions on 83 targets. The three rushing touchdowns on only 24 carries and three return touchdowns will be more difficult to repeat.

Hill's 2016 season is reminiscent of Cordarrelle Patterson's rookie season of 2013 (469 receiving yds, 158 rushing yds, nine total TDs). Patterson was similarly hyped to where Hill is now, and it didn't work out well for Patterson owners as he was largely irrelevant in 2014 and 2015 before re-emerging in 2016. Hill shouldn't disappear completely, but a step back in production seems likely.

Dak Prescott, QB, Dallas Cowboys (6.08)

Final stats: 3,667 pass yds, 23 TDs, 4 INTs; 282 rushing yds, 6 TDs

When Tony Romo was sidelined in the preseason, Prescott stepped into the starting role and never looked back. The former Mississippi State standout was exceptional, posting a sparkling 23-to-4 touchdown-to-interception ratio. Factor in his rushing production and lack of turnovers, and Prescott was one of the better fantasy quarterbacks last season.

The 2016 NFL Offensive Rookie of the Year will be hard-pressed to duplicate a nearly 6-1 TD-to-INT ratio, but he should remain a viable fantasy starter as long as he has a healthy Dez Bryant to throw to. With Ezekiel Elliott running behind an elite offensive line, the Cowboys' running game will continue to take pressure off Prescott.

Prescott is being drafted as the 8th quarterback off the board, ahead of more proven options like Cam Newton, Kirk Cousins, Russell Wilson and Matthew Stafford. If healthy, all four of those players could out-produce Prescott, who will likely see his turnovers increase in Year Two.

Rob Kelley, RB, Washington Redskins (N/A)

Final stats: 704 rushing yds, 6 TDs; 12 rec, 82 yds, 1 TD

Kelley, affectionately known as "Fat Rob," was a great story in 2016. He signed with the Redskins as an undrafted rookie free agent and eventually rose to the starter role, turning in a handful of solid performances down the stretch for fantasy owners.

His final stats don't jump off the page, but 601 of his 704 rushing yards and all six of his rushing touchdowns came after week 8. Washington's run game, which also included Matt Jones and Chris Thompson, was largely uninspiring, and always played second fiddle to its passing game with Kirk Cousins.

Kelley should factor into the 2017 backfield equation in Washington, and he could very well retain the starting job, but I expect the team to bring in either a free agent or a draft pick or two to compete for carries. Kelley's current price doesn't appropriately factor in the possibility for the Redskins to bring in other backs, making his price not palatable at this time

Honorable mention

Cody Kessler, QB, Cleveland Browns (N/A)

Final stats: 1,380 pass yds, 6 TDs, 2 INTs; 18 rush yds, 0 TDs

Cody Kessler was a part time starter for the Browns in 2016 and had some mild success, however, it is likely that the Browns draft a quarterback early in the NFL draft, relegating Kessler to the role of backup and worth nothing more than a stash in deep superflex dynasty leagues.