ALAMEDA, Calif. -- There was something gnawing at Jon Gruden as the Oakland Raiders reported to training camp last month in Napa.

He wondered aloud, "How much could Josh Jacobs eat? How long could the rookie running back stay at the table?"

It is a small sample size, no doubt, but Jacobs is hungry enough and, with only 251 career carries in college, the Alabama product should be able to stay at the table as long as he likes. And that suits Gruden just fine.

Or did you miss Jacobs' NFL debut at Arizona last week, when he started and gained 20 yards in his first three carries (he finished with 21 yards in four attempts)?

"He actually asked me what plays I wanted to run," a beaming Jacobs said after the game. "I knew I wasn't going to play too long, but he made it fun for me."

Fun? Jacobs was downright impressive with his vision, cutback ability, low center of gravity and in not shying away from contact.

In his first carry, with the Raiders in 22 personnel and in an I-formation, Jacobs took the handoff from Derek Carr and followed fullback Alec Ingold's lead block into the hole between center Rodney Hudson and right guard Jordan Devey for a 6-yard gain.

One snap later, the Raiders were in 12 personnel and Jacobs took the handoff and started left. He then planted his left foot and went right before hitting the line of scrimmage and finding a lane to the right of right tackle Trent Brown to pick up 8 more yards.

In his third carry, which was also the third play of the game, the Raiders were again in 12 personnel and Carr yelled at the line, "J-Lo, Phil Mickelson, J-Lo, Phil Mickelson" as the playcall with Oakland in the shotgun, Jacobs on Carr's right shoulder. Jacobs took the inside handoff and went left, eyeing the gap between Hudson and left guard Richie Incognito. But with the hole closing, Jacobs cut back to the middle, shooting the gap between Devey and Brown to gain another 6 yards.

And after a deep ball to Tyrell Williams -- and the accompanying 15-yard penalty on Arizona safety Budda Baker for leading with his helmet -- set up the Raiders at the Cardinals' 14-yard line, Jacobs again got the call. But Arizona won at the line of scrimmage and Jacobs was held to a 1-yard pickup up the gut.

And that was it for Jacobs.

"His vision," Carr said, when asked what stood out to him about the rookie. "You know I've been able to hand the ball off to some really good running backs. I've played with some fun ones -- MJD [Maurice Jones-Drew], D-Mac [Darren McFadden], Latavius [Murray], Marshawn [Lynch], Doug [Martin].

"Watching his vision was really impressive because I have the best seat in the house. I'm sitting back there playing 'Madden,' tell him to cut back and trying to run for him. He's right up there with those guys. For him to do that, especially in his first game? I think the thing that rubbed off on me the most about Josh was his want-to. It wasn't like, 'Oh man, this is my first game.' None of that. He was out there just trying to bully people. He was out there trying to run hard. He didn't care that he's a rookie. He's trying to prove that he's the best back in the NFL. That's his mindset. Really impressed ... with his demeanor and mentality."

Therein lies the quandary for the Raiders. Do they entrust the role of bellcow back to a guy with so little mileage on his body? Or is having so much tread left on his tires actually a good thing?

Because not only can Jacobs run the ball, he has also flashed great hands coming out of the backfield (he had a sick one-handed grab in camp) and shown he's not afraid to stick his nose in on pass protection.

So, does Jacobs' ascension make the veteran Martin expendable, or would the Raiders be wise to keep him around as an insurance policy? What about DeAndre Washington, who is having a better-than-solid camp, and Jalen Richard, who has been dinged up but offers a viable pass-catching option (as he showed last season, when he was tied for the team lead with 68 receptions)?

Then there's the issue at fullback. Ingold, an undrafted rookie who has been a revelation, or veteran and special-teams standout Keith Smith? Truly, how many backs can Oakland keep on cut-down day?

It's a good problem for Gruden & Co. to have ... for now. And one that Jacobs is eager to exploit, so to speak.

"I think it just speaks volumes on what they think of me and what they see in me," Jacobs said of the Raiders keeping his workload minimal thus far in the preseason. "So I am just honored to be out here and play football.

"[But] I was mad at myself because they should have been bigger runs. It was just knocking off the rust and things."

Indeed, Jacobs said the NFL game is starting to slow down for him. That will happen with more reps. And while he admits he has been hiding from the "Hard Knocks" cameras, the Raiders are hoping he is just as elusive for would-be tacklers.

Consider: Jacobs is only the second running back drafted in the first round by Gruden, joining Cadillac Williams with the Tampa Bay Buccaneers in 2005. And it was last year's trade of All-Pro edge rusher Khalil Mack to the Chicago Bears that landed Oakland the No. 24 overall pick, which was used on Jacobs.

"He won't be the feature back until he earns it," Gruden said on report day. "He's got to prove he can get up time and time again. These are car crashes, some of the hits these guys take. You got to be one tough guy. You got to be able to do it down after down. When you are tired and sore and beat up, you got to pick up a blitz, you got to beat a linebacker on a route, then you got to make a third-and-1 to win the game.

"So there's a lot of maturity that is going to have to take place because he's such a young and inexperienced player, but he's a great kid and got a lot of talent."