Trent Richardson viewed the Colts' running game as a three-man threat, right up until fellow running back Ahmad Bradshaw was placed on injured reserve this week with a broken left leg.

"I always say, 'A three-headed monster,'" Richardson said. "A lot of people say it's two-headed, but I say three-headed monster."

The third is Daniel Herron — the only remaining healthy running back on the active roster. Richardson is fighting an illness, but practiced Thursday one day after missing workouts.

The Colts (6-4) are looking for Herron to step up against Jacksonville (1-9) on Sunday with Bradshaw is expected to miss the rest of the season. Their best blocking tight end, Dwayne Allen, also missed practice for the second straight day with an injured ankle.

Offensive coordinator Pep Hamilton sees it as an opportunity for Herron to show he has what it takes, but wants to see a better mix on offense after the Colts finished with 19 yards on 17 carries Sunday night in a loss to the Patriots.

"I have to do a better job of making sure we maintain some balance," he said. "And find ways to attack what the opponent's doing. If they have seven or eight guys in the box, that's no excuse for us not to run the ball."

Richardson expects big things from Herron right away, but acknowledged the third-year running back is a bit inexperienced — he's carried the ball only 22 times this season. Still, Richardson said he and Bradshaw already know they can rely on him and expects Herron to get "that big score" against the Jaguars this weekend.

"A lot of people don't know how much work we put in with him," Richardson said. "To be that other guy and to be that guy we can trust when we're out and when we're tired or if something ever happened like this we know we can count on him."

Indy's injury woes for its backfield started long before Sunday's loss.

Vick Ballard was placed on injured reserved back in August with a torn Achilles and Richardson didn't play last month against Pittsburgh because of an injured hamstring. The Colts got 35 yards and a touchdown out of Bradshaw against the Steelers.

"The next man up filled that role," coach Chuck Pagano said this week. "So it's next man up. So it's Trent and (Herron) and whoever."

Bradshaw set a career-record with two receiving touchdowns against the Eagles back in September and has six touchdown receptions on the year. And just five weeks into the season, Bradshaw surpassed 1,000 career rushes.

He and Richardson total more than 500 scrimmage yards and the Colts are one of four teams (Cincinnati, Philadelphia, and Washington) in the NFL with two players to reach that total.

If the Colts need a third running back, though, Pagano said they will make moves to add one.

"We've got guys in the building, and roster mechanics are what they are," Pagano said. "We'll always check and see if anybody's available on the street. If there's somebody available, we look at guys on the street, we look at guys on our roster, and then we make a decision."