Matt Cassel is back under center as the Chiefs' starting quarterback.

KANSAS CITY, Mo. (AP) — Matt Cassel is back under center as the Chiefs' starting quarterback.

Cassel will start Thursday night's game at San Diego because Brady Quinn has not been cleared to practice due to a concussion, coach Romeo Crennel said Tuesday. Ricky Stanzi will be the backup.

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 sustained his own concussion against the Ravens. Quinn took over for a loss 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 and 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 sustained a "head injury" and was unavailable.

Cassel played the rest of way, completing 20 of 30 passes for 218 yards with a touchdown and an interception. It was the 10th pick that he's thrown in parts of six games this season.

"He prepared mentally as if he was going to play, and that allowed him to come into the game and move us a little bit and spark us," Crennel said, "but still some of the same things that have been biting us all year continued. We have to eliminate those things."

Crennel said that doctors haven't told him how long Quinn will be out, so Cassel — who signed a $63 million, six-year deal in 2009 — is back in his familiar role as the starter.

"Sometimes a second chance energizes a guy," Crennel said. "I think Matt is a team guy. 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. That's what he did during (last) week and that's what he's going to do this week."

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, according to STATS LLC.

They've also turned the ball over a league-leading 25 times after four turnovers Sunday.

Cassel has responsible for two more on 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."