KANSAS CITY, Mo. -- Matt Cassel is back under center as the Chiefs' starting quarterback.
Fullback Nate Eachus has also been ruled out with a concussion.
"They will continue with their testing and evaluation and will return when the doctor releases them, and lets us know they're OK," Crennel said.
Cassel was the starter the first five games of the season before he suffered a concussion Oct. 7 against the Ravens. Quinn took over in a loss Oct. 14 to Tampa Bay, and Crennel made the move permanent during the bye week in hopes of igniting a struggling offense.
He never got much chance to see whether it worked.
Quinn was hurt in the first quarter of Sunday's 26-16 loss to the Oakland Raiders, though it's unclear when the injury occurred. He was sacked by Rolando McClain, then took another shot while floating a pass that was picked off by Matt Giordano.
Quinn left the field and went down the tunnel leading to the locker rooms, and Crennel said a trainer told him at that point that Quinn had suffered a "head injury" and was unavailable.
Cassel played the rest of the way, completing 20 of 30 passes for 218 yards with a touchdown and an interception. It was the 10th pick he's thrown in parts of six games this season.
"I'm excited to be playing," Cassel said. "As I said last week, my approach doesn't change. I'm going to go out (and) I'm going to work hard. It's unfortunate for Brady, but at the same time, we have to move forward. It's a short week. Preparation has to be put in."
Cassel, who signed a $63 million, six-year deal in 2009, said he doesn't harbor any ill will toward Crennel over his benching. Cassel also said he doesn't find it uncomfortable that he's regained the starting job on a temporary basis almost by default.
"I don't look at it that way," Cassel said. "No matter what my position is, I'm going to be the same guy, and I think that's the reason I was able to go in there and execute at a high level Sunday. I was ready and my approach was the same."
Cassel said he wasn't sure whether coming off the bench changed his perspective, but his coach indicated that the decision ultimately may help Cassel from a mental standpoint.
"Sometimes a second chance energizes a guy," Crennel said. "He wants to do well, and whatever role he was in, he didn't like it, but he said, 'I'm going to do my job. I'm a Chief, and I'm going to be ready if I'm called on.' "
The Chiefs (1-6) could certainly use a little energy. They've lost four straight overall and still have not led in regulation this season, making them the first NFL team since at least 1940 to play seven games without taking an offensive snap with the lead.
They've also turned the ball over a league-leading 25 times, including four turnovers Sunday.
Cassel was responsible for two Sunday when he fumbled a snap early in the second half that led to a field goal by Oakland. He's committed 16 turnovers by himself, which puts him ahead of all but three teams in the league: Dallas, Buffalo and Philadelphia.
"He knows he's going to have to play, and he's going to do the best he can," Crennel said. "We have to help him as well, and if we all take the same attitude we'll be successful."
Offensive coordinator Brian Daboll said it helps, given the short week, that Cassel was the starter when the Chiefs played San Diego earlier this season. Cassel committed three turnovers in that game, but he'll be familiar with the Chargers' schemes and personnel.
"He's preparing himself like a pro to be ready to go in this football game," Daboll said. "He's got a good attitude, he's taken Brady being put ahead of him well. If you ask him, I'm sure he's anxious, excited for the opportunity."