Indianapolis Colts running back Vick Ballard was placed on injured reserve Friday, a day after tearing his right ACL during practice.
Ballard rushed for a team-high 814 yards as a rookie last season and helped the Colts in their surprising run to the playoffs.
Colts coach Chuck Pagano said Ballard was injured after he planted his foot on the ground and his knee gave out.
"We feel awful for Vick," Pagano said. "We all know Vick was headed toward having a fabulous year. We know and understand injuries are part of this game. We'll deal with it. No one quite handles adversity like this team."
Ballard started the season opener against the Oakland Raiders and rushed for 63 yards on 13 carries.
With Ballard out for the season, Ahmad Bradshaw and Donald Brown will assume most of the carries.
Bradshaw rushed for 1,015 last year with the New York Giants and is averaging 4.6 yards per carry for his career.
"It's bad news," Bradshaw said. "He's been working hard all year. It's just a freak accident. My heart goes out to Vick as hard as he's worked for this year. Our plan was to be that 1-2 punch.
"I'm set to go any time. That's what happens when this happens. I'll be ready."
Brown, a first-round draft pick in 2009, led the team in rushing in 2010 (497 yards) and 2011 (645).
"It's next man up, we have to keep rolling," Brown said. "Since day one that's been the mindset. My preparation hasn't changed. I'm ready to roll, ready for Sunday. I'll be ready for any opportunity that comes my way."
The Colts host the Miami Dolphins.
To take Ballard's spot on the 53-man roster, the team promoted running back Kerwynn Williams from the practice squad. Williams was the Colts' seventh round pick in this year's draft out of Utah State.