(A series looking at the top players at various positions leading up to the 2015 NFL Draft, set for April 30-May 2.)

When it comes to running backs in the NFL Draft, it's not so much a question of who, but when? Identifying the top prospect here has taken something of a back seat to guessing which round will contain the first running back to come off the board.

There hasn't been a running back picked on the first round of an NFL Draft since 2012, when Trent Richardson, Doug Martin, and David Wilson all were among the first 32 players selected. This could have been the year to break that streak, but the best prospect at the position – Todd Gurley – is still rehabilitating his knee after major surgery to repair a torn ACL.

TODD GURLEY
He didn't allow the knee to be examined by the gaggle of medical personnel assembled at the NFL Scouting Combine, and so teams interested in Gurley figure to bring him in for some tests of their own. Some of the good things about Gurley's college career include the fact he had only 510 career carries at Georgia, but he averaged 6.4 per carry and scored 36 rushing touchdowns. Also, he'll be a 21-year-old rookie in the NFL, 61.9 percent of his rushing yardage in 2014 came after contact, and he fumbled only three times in those 510 career carries. The bad includes an injury history that started with a three-game absence for an ankle injury in 2013 and that torn ACL last fall.

MELVIN GORDON
Like most Wisconsin running backs, Gordon got his hands on the football a lot, but he really had only one season when he was the bell cow. In a four-year career that included 631 carries, Gordon's heaviest workload came in 2014 when he had 343 attempts and averaged 7.5 yards and scored 29 rushing touchdowns on the way to winning the Doak Walker Award. Gordon capped off his senior season by winning the Outback Bowl MVP award based on goring Auburn for 215 yards on 34 carries (7.4 average) and three touchdowns. The primary issue with Gordon is whether he can be a three-down back in the NFL, because neither his blocking nor his receiving skills are good enough right now.

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TEVIN COLEMAN**
Under the radar because he played his college football at Indiana, Coleman's parents were born in Liberia, where his great-great grandfather (William David Coleman) served as that country's president from 1896-1900. In his three seasons with the Hoosiers, Coleman carried 452 times and had a 7.1 average with 28 touchdowns. A team captain, Coleman averaged 142.6 yards per game over the 2013-14 seasons to rank second nationally in that category. The fact he had to carry his team, and the fact he rolled up the statistics he did without great teammates surrounding him have to be considered factors in his favor as he transitions to the NFL.

DUKE JOHNSON
Whichever NFL teams picks Johnson will be getting a player who should be able to contribute right away on special teams as a returner. In 2012, Johnson posted 2,060 all-purpose yards for the Miami Hurricanes, and included in that total was 27 kickoff returns for 892 yards (33.0 average) and two touchdowns. On the plus side, Johnson had only six fumbles in 637 career touches in college, but he also missed time with ankle, hamstring and concussion injuries. He would be a great fit for an outside-zone scheme, because Johnson is a decisive one-cut runner. A question is how much punishment can he withstand, especially since he's 5-foot-9, 207 pounds.

JAY AJAYI
After tearing an ACL and missing the 2011 season, Ajayi totaled 678 carries for 3,796 yards (5.6 average) and 46 rushing touchdowns. Last season, Ajayi became the first player in FBS history to finish a year with 1,800 rushing yards and 500 passing yards. He showed himself to be a willing blocker in college, which will enhance his value as a three-down back, and Ajayi also was pretty good in short-yardage and goal-line situations.

THE 2014 NFL DRAFT, RB STATISTICS
Number drafted: 22
Picks by round: 0 in the first; 3 in the second; 5 in the third; 6 in the fourth; 0 in the fifth; 5 in the sixth; 3 in the seventh
Highest pick: Bishop Sankey, Washington, Round 2, 54th overall by the Tennessee Titans
Biggest impact: Jeremy Hill, and by a significant margin. He played in all 16 games, with eight starts, for a Bengals team that qualified for the playoffs. Hill finished with 1,124 yards rushing via a 5.1 average, and he scored nine touchdowns.