Cassel will start Thursday night's game at San Diego because Brady Quinn has not been cleared to practice due to a concussion, regaining the starting job - at least for one week - that he lost after a dismal five-game stretch to start the season.
"They will continue with their testing and evaluation and will return when the doctor releases them, and lets us know they're OK," said Crennel, adding that doctors haven't told him how long Quinn might be out.
Cassel was scuffling along until he sustained his own concussion a few weeks ago against Baltimore. 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.
"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 may ultimately 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, 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."
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 the 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."