INDIANAPOLIS -- With the starting running back position for the Indianapolis Colts solidified by Frank Gore, the most intriguing battle could be between Vick Ballard and Daniel "Boom" Herron for the primary backup spot.

Herron signed his exclusive rights tender on Wednesday. He now will officially battle Ballard to back up Gore.

Herron moved into a more prominent role in the backfield last season when Ahmad Bradshaw went down with a broken leg in Week 11. He had a 49-yard touchdown run, the longest run of the season by a Colt, against Washington in Week 13.

Herron replaced the now-departed Trent Richardson in the starting lineup in Week 16 at Dallas. Herron started all three playoff games, too. His biggest issue last season was his inability to hold on to the ball (two lost fumbles). He finished with 351 yards on 78 carries last season.

Ballard had a promising rookie season in 2012, rushing for 814 yards on 211 carries. But he has only played one game since that season. He tore his ACL in practice after Week 1 in the 2013 season and then he tore his Achilles during training camp last season. So the biggest question mark surrounding Ballard is whether he can stay healthy.

"[Ballard] is doing well," coach Chuck Pagano said. "Vick is a guy who is not going to miss a treatment. He's living in that training room, that weight room. He's working his tail off to get back. He's on schedule. That return date, again, our docs will communicate with us. When they feel like he's 100 percent and ready to go, he'll get back on the grass and let us know."

The Colts also have Zurlon Tipton, who was Herron's backup, and Jeff Demps on the roster at running back. They like the depth at running back in this year's draft, so selecting a player is definitely a possibility.