Carr was hurt in the third quarter of Oakland's 16-10 loss to the Denver Broncos on Sunday. His injury was initially described as back spasms.
"I think he just got roughed up there. He got tackled. Something caused his back to spasm up," Del Rio said, per Conor Orr of NFL.com. "I didn't see much that would indicate the he would be injured. His back spasmed up."
Said Carr: "I tried to come in here and have them work on it. I tried to throw and all those kind of things, and it just wasn't ready yet. I tried to do anything I could to be back out there. I just couldn't."
As Ian Rapoport of NFL Network and Michael David Smith of Pro Football Talk noted, Carr's timetable to return could be short:
If that timetable is extended, however, it would be a major blow for an Oakland team that has Super Bowl aspirations but has opened the season just 2-2. The Raiders had to adjust to life without Carr at the end of last season after he broke his fibula in Week 16. The team then lost its next game against the Broncos, missing a chance for a first-round bye in the playoffs, and lost at the Houston Texans in the Wild Card Round.
With Carr sidelined, the team will turn to EJ Manuel at quarterback. Manuel finished 11-of-17 for 106 yards with an interception after Carr was injured Sunday.