SEATTLE -- When the Seattle Seahawks signed Eddie Lacy to a one-year deal in free agency, the assumption was that he'd combine with incumbent starter Thomas Rawls to fill the leading roles in the team's restocked backfield.

The Seahawks may need that to be the case after all after rookie standout Chris Carson, who had taken over the starting role, suffered an ankle injury Sunday night. It looked serious and was repeatedly characterized as such by coach Pete Carroll following the Seahawks' 46-18 win over the Indianapolis Colts.

"Chris Carson hurt his ankle, and he hurt it pretty significantly," Carroll said. "We had a couple different reports here, but now we know that he did hurt his ankle significantly. We’ll see what that means when we get all the MRIs and all that stuff done."

Carroll gave a wait-and-see response when asked about the severity of Carson's injury.

"Let us get through all the stuff. There’s no reason for me to try to tell you what I’ve heard," Carroll said. "He's got a significant ankle [injury]. We don't know if he needs surgery or any of that kind of stuff. We don't know that yet. But it's significant."

Carson went down midway through the fourth quarter when his left ankle was pinned awkwardly underneath a defender while he was being tackled. Members of the team's medical staff applied an air cast to his left leg while Carroll, perhaps Carson's No. 1 fan, leaned over him. Most of the Seahawks ran over from the sideline and surrounded Carson before he was taken off the field on a cart.

That's how much Carson has meant to the Seahawks over their first four games. He has been Seattle's leading running back in terms of carries and playing time, while Rawls and Lacy have played sparingly, a scenario that few could have envisioned when Seattle drafted Carson in the seventh round.

Rawls and Lacy entered Week 4 with only 24 offensive snaps and 10 carries between them through the first three games. An ankle injury that sidelined Rawls in the opener against the Green Bay Packers and limited him in Week 2 against the San Francisco 49ers was part of the reason for that. It hasn't helped that the Seahawks' struggles on offense have forced them into way more throwing situations than they'd prefer, either in order to make up lost yardage on early downs (that happened repeatedly in Week 1) or to make up a big deficit (as was the case last week against the Tennessee Titans). That hasn't left many carries to go around.

But Carson has simply been the best option. Now he could be sidelined for a while.

"Hopefully he's OK," right tackle Germain Ifedi said. "I think I saw him waking around [in the locker room], so hopefully it wasn't too bad. We love having him in there. Tough to see him down on the field like that, but hopefully it's not too bad and he's able to come back sooner rather than later."

Softening that potential blow a bit is the fact that the Seahawks' running game finally came alive against the Colts, racking up a season-high 194 yards thanks to contributions from several players in addition to Carson, who gained 42 yards on 12 carries before his injury. With Rawls inactive -- he was a healthy scratch -- Lacy played for the first time since Week 1 and led the team with 52 yards on 11 carries. J.D. McKissic, active for the first time this season, added 38 yards and a touchdown on four carries (he also caught a 27-yard touchdown pass). McKissic looks like he can help the Seahawks even when C.J. Prosise returns from an ankle injury that sidelined him in this game.

Lacy broke off runs of 7 and 19 yards right after Carson went down. His next carry went for 2 yards but could have been more had he not got tripped up while running through a hole on the left side.

"He looked good," Carroll said. "He stumbled that one time. I would have loved to see him stay on his feet there so he could do some damage downfield. I thought he looked exactly like we hoped. He looked big and strong and tough and very decisive and all, and he just needs to get some more carries to get going. I thought that was great."

The rest of Carroll's answer was seemingly a hint that Rawls will be the first option in Carson's absence, assuming there will be an absence. But even if that's the case, Lacy will still factor in as well.

"With Chris being banged up, we’re so fortunate to have Thomas ready to jump back out there," Carroll said. "We’re just lucky that we have a guy like that who’s ready to go and is healthy and all that.”