Washington Redskins coach Jay Gruden said quarterback Robert Griffin III sustained a concussion in the team's 21-17 exhibition victory over the Detroit Lions.
Gruden said after Thursday night's game that he has "no idea'" how long Griffin will be sidelined.
The coach added that he "would imagine, depending how long" Griffin is out, "he's still going to be our starter."
Griffin left in the second quarter after getting sacked on consecutive plays. He also got a stinger in his right shoulder, but Gruden said that should be OK.
The Redskins initially said Griffin was questionable to return, but later ruled him out for the rest of the game.
Griffin was hurt early in the second quarter when he fumbled the ball without contact while scrambling, then got landed on by defenders. He stayed down on the field for about 5 minutes and was tended to by trainers, while some teammates kneeled nearby.
After Griffin got up, he walked slowly to the sideline, then kept going until he left the field and entered the tunnel that leads to the Redskins' locker room. Griffin gave a thumb's up to spectators as he trudged away.
The 2012 NFL Offensive Rookie of the Year completed 2 of 5 passes for 8 yards and was sacked three times and hit hard on several other occasions while playing four series.
He fumbled twice. The first was recovered by the Redskins after a strip-sack on their opening possession. The second came on Griffin's final play and was recovered at Washington's 11 by Detroit, which then needed four plays to score a touchdown for a 10-0 lead.
It capped a rough week for Griffin, who caused a national "firestorm," to use his word, with comments in a television interview about striving to be the best quarterback in the NFL. He complained that his remarks were taken out of context, saying after practice Tuesday: "It's unfortunate that my name keeps getting used for headlines for people to click on stories. But I know what I want to be, I know what I strive to be and I'm not going to make any apologies for that."