The ability to come in and contribute right away might not be the end game on evaluating a rookie prospect, but it does give his team a much-needed boost, normally at a position of need. This year has seen several first-year players on both sides of the ball come in and make a large impact from day one. Here, we're going to take a look at the top rookies on the defensive side of the ball—and this year's winner wasn't even selected in the first round of the 2015 NFL Draft.

Winner

Ronald Darby, CB, Buffalo Bills

Darby wasn't the most heralded cornerback from this draft class heading into the season, and with another rookie leading the league in interceptions and pass breakups, it's fair to say that he isn't really getting the credit he deserves at the end of the year. In all honesty, we didn't see a better performance by a rookie all year, though. Darby stepped right in and started from day one, instantly becoming Buffalo’s best cover corner.

He allowed just four touchdowns all season, all of which came in a two-game span against the Kansas City Chiefs and Houston Texans. Darby allowed just 660 receiving yards throughout the year, he was tied for fifth amongst cornerbacks with 13 pass breakups, and finished the year with the ninth-highest coverage grade for corners. There is still plenty of room for Darby to grow, with just two interceptions over the course of the year, and a reception of 20 yards or more allowed in six games in 2015. Nevertheless, it was an impressive rookie campaign that has Bills fans eager for the future, and rightfully so.

First runner-up

Leonard Williams, DI, New York Jets

Coming into the draft, we knew that USC's Leonard Williams had a bright NFL future ahead of him as a run defender, but there was a concern that he lacked the ability to be disruptive as a pass rusher. His rookie season might not have been as dominant as rushing the quarterback, but he showed that it's not a true weakness in his game either, registering four sacks, 19 hits, and 30 hurries in his debut season. Predictably, Williams’ real strength was against the run, and even as a rookie, we saw him prove to be far too much for some veteran offensive linemen in the league. Over the course of the year, he registered 35 tackles resulting in a defensive stop, the 14th-most of any defensive interior player this season.

Second runner-up

Byron Jones, DB, Dallas Cowboys

Dallas' first-round draft pick showed his versatility in 2015, spending time at outside cornerback, in the slot, and at safety in the first year of his career. He was solid against the run, but it was his play in coverage that earned him a spot as a top-three defensive rookie, where he notched six pass breakups over the course of the year. The Cowboys had the foresight to uncover his versatility, and by the end of the season, Jones had played 249 snaps as an outside corner, 185 as a slot corner, 270 as a free safety, and 186 as a strong safety or extra linebacker in the box. With front offices now salivating for that chess piece player who can move all across the defensive backfield, Jones thrived in his varying role in the Cowboys' defensive backfield.

Third runner-up

Marcus Peters, CB, Kansas City Chiefs

From a statistical perspective, it might seem incredibly harsh that Peters, who leads the league with eight interceptions and 17 pass breakups, is only our third runner-up for the Defensive Rookie of the Year award. While soaking in the stats, you have to take into consideration what he allowed in coverage. The former Washington Huskie gave up 939 receiving yards, the third-most of any cornerback in the league this season (not just rookies), and allowed the second most touchdowns in the league, with eight.

Both of these numbers are inflated and impacted because no cornerback in the league was targeted as often as Peters, who saw 137 passes thrown his way this year, and the truth of his performance is somewhere in the middle. He isn't the best cornerback in the league because he had the most interceptions, and he's certainly not the third-worst because he allowed 939 yards. His 2015 season, summed up, was a very solid first year in the league, and one from which he has a platform to become a household name.

Fourth runner-up

Adrian Amos, S, Chicago Bears

A fifth-round draft pick out of Penn State, Amos is the only player not drafted in the first two rounds of the draft to crack our top five this year. He stepped straight into the Bears' starting lineup from day one, and played 1,046 snaps over the course of the year. A reliable player in coverage, Amos’ real strength was against the run, with 10 of his 25 tackles there resulting in a defensive stop. He added nine defensive stops in coverage, and notched a sack on one of his 10 pass rushing attempts, all adding up to a solid rookie season in Chicago.

For more PFF awards, visit the following pages:

All 2015 PFF Awards

Dwight Stephenson Award (best player in NFL)

Most Valuable Player

Offensive Player of the Year

Rookie of the Year

Comeback Player of the Year

Breakout Player of the Year

Most Improved Player of the Year

Best Pass Rusher

Best Run Defender

Best Coverage Defender