DAVIE, Fla. -- The Miami Dolphins will take the field in less than three weeks for their regular-season opener. But coaches, players and fantasy owners still don't know who will be their starting running back.

Miami's closest position battle this summer involves second-year tailback Jay Ajayi and former Pro Bowler Arian Foster. It's been back and forth throughout training camp, with each player having their good and bad moments in practices and preseason games.

Foster, who is returning from an Achilles injury, mostly looked good in practices but rushed for minus-5 yards on two carries in his preseason debut against the Dallas Cowboys. Ajayi hasn't flashed as much in practice, but has 25 rushing yards in two preseason games.

Both running backs will play during the regular season, but only one can start. Right now it is anyone's guess who that will be.

"In my mind right now I look at myself as the starter," Ajayi said Monday. "I'm not worried about what anything else is. That's just my mentality to go out there as the No. 1 guy with that mindset, and when I step on the field, make plays."

Dolphins head coach Adam Gase is taking a patient approach with his running backs. He hasn't given an indication either way, and the team has been diligent to give both players split reps with the first team throughout training camp.

Gase made a decision to sit Foster in the preseason opener and only gave him a pair of carries this past Friday against Dallas.

"We didn't have much to evaluate," Gase said. "The couple of carries that he had, there wasn't much there. He tried to create a couple things on his own. Hopefully we can get him going a little bit this week. Obviously I don't want to overdo it with him but I'm pretty sure his résumé speaks for itself."

Look for both running backs to get more carries Thursday against the Atlanta Falcons. The third preseason game could decide this close race between Foster and Ajayi, because many starters won't play in the fourth preseason game.

This week could be the final time Miami plays its offense at full strength before traveling to play the Seattle Seahawks on Sept. 11.

"That's what preseason is for," Foster said. "The preseason is for knocking the cobwebs off, getting back in your grove, seeing things that you haven't seen in a while and getting a feel for the game again."