ANDERSON, Ind. -- Everybody knows about running back Frank Gore. The same can't be said when it comes to naming his backup.

Everybody knows T.Y. Hilton, Phillip Dorsett and Donte Moncrief will be the Indianapolis Colts' top three receivers. But what about after that? That's a mystery. The same can be said after Dwayne Allen and Jack Doyle at tight end. Can't forget about the safety position, either.

The Colts have talent at their skill positions, but they also have depth concerns at the positions listed above.

"We have three guys at wide receiver [who] we know about and then we have a bunch of guys [who] have talent, can run, have height, weight and speed and can catch and do all those things, but they have to go do it now," coach Chuck Pagano said Thursday. "Those guys are all going to compete for that spot, and that's what training camp is all about. We've got talent, we've got guys that are capable, they are just unproven."

The Colts have nine receivers who don't have the last names Hilton, Dorsett or Moncrief. Brian Tyms and Josh Boyce are the only ones who have a reception in the NFL. The Colts have routinely kept around five receivers during the regular season.

There's no clear-cut favorite to be Gore's backup, even though the team is high on undrafted free agent Josh Ferguson out of Illinois. Gore, 33, had 227 more carries than his next-closest teammate last season.

The Colts signed tight end Chase Coffman earlier this week, but they can't afford to have Allen go down with an injury because that only leaves Doyle and Coffman with any experience.

Safety is another position where they've suddenly become thin. The starter, Clayton Geathers, is out with a foot injury and rookie T.J. Green did not practice Thursday after suffering a left calf strain in practice Wednesday.

The Colts could sign a veteran free agent -- like safeties Donte Whitner or Antrel Rolle -- to try to add depth. But the team is moving away from quick fixes when it comes to plug-and-play players, because it hurts the development of the young players on the roster. Of course, that philosophy could change if it gets close to the regular season and they need roster help because the younger players aren't producing the way the team believes they should be. The Colts are optimistic Geathers will be back from his foot injury in plenty of time.

"After learning from last year, you better have depth," Pagano said. "It's 39 guys. I finally got a chance to watch the NFL Films 2015 Colts [season recap segment], and you tend to forget those things and didn't realize we had that many guys that actually missed time.

"That is why we challenge our players, as a coaching staff, to every man -- regardless of where you are lining up right now -- prepare as a starter because you never know when that opportunity is going to come."