Free agency, the combine and the owners meetings are in the rearview mirror, and the NFL draft is just a few days away. But it’s time to get back to football on the field. Here’s a quick look at the Oakland Raiders as they open their offseason workout program Monday.

Biggest challenge facing coach Jack Del Rio: Making sure all the pieces fit. The Raiders may have won free agency with the additions of offensive lineman Kelechi Osemele, outside linebacker Bruce Irvin, cornerback Sean Smith and free safety Reggie Nelson, but how they vibe with the rest of a team on the rise will tell Del Rio’s tale. The players have all said the right things so far, and getting together as a unit for the first time is exciting. But how they interact and buy into Del Rio’s vision is the test.

Player on the rise: Receiver Seth Roberts finished with the fourth-most catches in Oakland last year, behind Michael Crabtree, Amari Cooper and Latavius Murray, with 32 for 480 yards, though his yards per catch average of 15 yards led the team. Roberts is not your typical slot receiver in the Wes Welker/Percy Harvin/Danny Amendola mold; Roberts is a bigger target at 6-foot-2 and 195 pounds. But he showed his explosiveness, especially with a game-winning touchdown catch against the Baltimore Ravens (one of his five TDs) and a clutch 33-yard tipped pass to himself against the San Diego Chargers on Christmas Eve.

David Amerson's four interceptions were the second most on the Raiders last season. Jim Brown/USA TODAY Sports

Player who has the most to prove: It starts and ends with cornerback D.J. Hayden, about whom questions have already surfaced as to whether the former first-round pick even makes it to training camp on the Raiders' roster. Drafted at No. 12 overall in 2013 despite having nearly died on the practice field the previous fall, Hayden has had a rough go of it in his first three NFL seasons. The defensive back who gave the Raiders visions of Mike Haynes has just three interceptions in 34 games.

Surprise player to watch: David Amerson’s emergence last season was more shocking than surprising, given that he was a waiver claim from Washington prior to Week 3 and his four interceptions were the second most on the team. He also had his second career pick-six, taking a pass from the Kansas City Chiefs’ Alex Smith 24 yards to the house in the season finale. Now, it will be interesting to see if he can hold off Hayden for a starting cornerback position and, if so, how he responds to playing opposite free-agent veteran Sean Smith, a true No. 1 corner.

Position that should be addressed in draft: Let’s see, the Raiders already addressed offensive line, linebacker, cornerback and free safety in free agency, and they already have franchise players at quarterback, receiver and defensive line/outside linebacker. Still, adding another safety (Karl Joseph?) to go with Nelson and Nate Allen, and/or a running back (Ezekiel Elliott?) to pair with Murray, who rushed for 1,066 yards and six touchdowns and caught 41 passes for 232 yards, would not be a bad idea, either. But adding another run-stuffer to the interior defensive line (Sheldon Rankins? Robert Nkemdiche?) early might be the best call of all.