ALAMEDA, Calif. (AP) With a starting running back that hasn't had more than 100 carries in a season since entering the NFL, the Oakland Raiders know they will likely have to count on some of their backups to generate a consistent running game this season.

The race to be Latavius Murray's backup remains wide open heading into the third exhibition game Sunday against Arizona, with no one able to seize the job either because of injuries or ineffectiveness.

Trent Richardson and Roy Helu Jr. were signed in the offseason to fill that role. But Richardson is averaging just 2.3 yards per carry and lacks the burst that made him the third overall pick in the 2012 draft. Helu finally returned to practice this week after missing more than three weeks with an injury.

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That has helped open the door for undrafted free agent Michael Dyer to make his case for the job. After struggling in the preseason opener, Dyer ran for 45 yards on 12 carries last week in Minnesota.

''I feel like I got my confidence up because I was able to get into a rhythm and up to game speed,'' Dyer said. ''Going from the first game to second game you try to build confidence and get the offense down. By the third game you should be able to have control of the game and play hard and fast.''

It's been quite a road for Dyer, a freshman star at Auburn who helped the Tigers win the 2010 national championship game by rushing for 143 yards in the title game against Oregon to the NFL.

Dyer had a pair of 1,000-yard seasons at Auburn before being suspended for the bowl game his sophomore year and then transferring to Arkansas State. He got dismissed from the team before ever playing at Arkansas State.

Dyer earned his associate degree at Arkansas Baptist College before playing his final two years at Louisville, where he rushed for 704 yards in 17 games. Dyer was not drafted and didn't earn an NFL contract until after impressing the Raiders enough on a tryout at rookie minicamp in May.

''He runs hard and is very determined,'' coach Jack Del Rio said. ''He has a little juice to him. He is showing up, making people miss and accelerating through some tackles and doing a pretty good job. He has come an awful long way from when he first came in here in the spring as a late addition.''

Richardson had a much easier entrance into the NFL after starring at Alabama, but is with his third team in four years and could be running out of time after already being traded by Cleveland and cut by Indianapolis.

Richardson has gotten most of the second-team work so far. He is far from assured of staying in that role once the season begins.

''He's got to earn it,'' Del Rio said. ''He's got to be good enough as a backup. We've got to see some of the explosiveness that he had when he was a young man playing for the Crimson Tide there. He's got to have a role on special teams and he's got to be able to help us win on Sundays. That's the challenge for him right now.''

Despite missing most of training camp, Helu figures to have a role because of his skill as a third-down back able to pick up blitzes and make big plays as a receiver out of the backfield.

Helu has 129 catches in 48 career games and was tied for fourth in the league among all running backs with 31 third-down catches the past two seasons. He hopes for a bigger role in Oakland, but knows Murray will be the lead back.

''I think most everybody's desire as a running back is to get involved with a rhythm because that's such a big part of the position,'' he said. ''At the same time, my last couple of years I've been in the NFL I was designated in that role. Whatever role I'm playing and whatever that looks like I'll do my best at it.''

The wild card at the position is Taiwan Jones, who brings electrifying speed to the position after spending the past two seasons at cornerback. Jones has rushed for 27 yards on five carries in two preseason games.

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