DAVIE, Fla. (AP) Miami Dolphins practice began Monday with first-team running back Jay Ajayi on the sideline, riding a stationary bike and making it harder for the team's ground game to get rolling.

Ajayi was held out of the first workout in full pads because of knee trouble, an issue that hurt his draft status in 2015. He and the Dolphins said the latest injury isn't serious, but his absence comes at a position already filled with uncertainty.

''The more you can keep that group working together, the better,'' coach Adam Gase said. ''But this is the NFL, right? This is what happens. Next man up.''

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Not quite ready to be the next man up is four-time Pro Bowler Arian Foster, making a comeback from Achilles tendon surgery last fall. Foster signed with Miami last month and was cleared to join practice Sunday, but the Dolphins plan to phase him in slowly.

Among the other running backs, Damien Williams reported out of shape and has yet to practice, and Isaiah Pead limped off the field midway through Monday's drills. Also contending for playing time are third-round draft pick Kenyan Drake and veteran Daniel Thomas.

Ajayi said he suffered a bone bruise Sunday which caused swelling in his left knee. Gase noted it was Ajayi's other knee that required surgery for a torn ACL in 2011, when he was at Boise State.

''I think he was worried it was more than what it was,'' Gase said. ''Thankfully it's on the other leg.''

Even so, Ajayi joined Gase's pals Peyton Manning and Wes Welker - among others - as a spectator at the workout.

Ajayi said he expects to be back to normal in a couple of days, resuming his pursuit of the starting job as a successor to Lamar Miller, who departed via free agency during the offseason.

Ajayi played in only nine games last season because of a rib injury, and rushed for 187 yards in 49 carries as Miller's backup.

''Opportunities like this are very limited to be a starter in the NFL,'' Ajayi said. ''I'm excited.''

He said he's more comfortable and confident than he was as a rookie.

''Last season I went through some ups and downs,'' he said, ''but I learned a lot.''

A fifth-round draft pick, Ajayi likely would have gone higher if not for lingering concerns about his right knee. He rushed for 3,796 yards and 50 touchdowns in college, and as a senior became the first player in FBS history to gain at least 1,800 yards rushing and 500 receiving in a season.

First-year coach Gase was impressed by the 229-pound Ajayi in offseason drills and believes he could emerge as the biggest surprise of training camp.

Or perhaps it will be Foster. Injuries limited him to 25 games in the past three years with the Houston Texans, and he played in just four games last season, averaging 2.6 yards per carry.

But Foster said he feels healthier than 12 months ago. When he took the conditioning test at the start of camp, Gase received this appraisal of Foster from an assistant coach: ''He absolutely annihilated it.''

''I think he was trying to prove a point to us, like, `I'm ready to go,''' Gase said.

Like Ajayi, Foster knows there's a job available.

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