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KANSAS CITY, Mo. (AP) — Chiefs quarterback Matt Cassel has been ruled out for Sunday's game at Tampa Bay with a concussion, and Brady Quinn will start an NFL game for the first time since the 2009 season.

Coach Romeo Crennel said Thursday that doctors have not cleared Cassel after he was hurt in the fourth quarter of a 9-6 loss to Baltimore last Sunday. Cassel has only spent a short period of time at the practice facility this week and it's unclear how long he might be out.

"We're trying to put the focus on preparing for Tampa Bay. We got a start on that yesterday," Crennel said. "We're going to try to be competitive and give ourselves the best chance to win." Quinn, a former first-round draft pick of the Cleveland Browns, has been taking repetitions with the first team in practice this week. Ricky Stanzi will serve as the backup.

Cassel struggled mightily even before getting hurt, and it was possible that a change in quarterback would have been made eventually. Cassel had been completing just 58.5 percent of his passes and thrown nine interceptions against five touchdowns this season.

Quinn was perfect on three pass attempts on the Chiefs' final series against Baltimore, but had a go-ahead touchdown pass wiped out by offensive pass interference. Kansas City was forced to kick a field goal, and the defense was unable to get the ball back in the closing minutes.

Quinn hasn't started a game since Dec. 12, 2009, when he was still with the Browns. He threw for just 66 yards and a pair of interceptions in a 41-34 victory at Kansas City, but wound up getting hurt in the game. He landed on injured reserve afterward, was eventually traded to the Denver Broncos and then landed back in Kansas City this past offseason.

"I'm still just preparing and staying ready," Quinn said. "One of the biggest differences for me is you get to practice more instead of taking more mental reps, so that's an advantage." Crennel said he won't necessarily tailor the game plan for Quinn, partly because he believes that the former Notre Dame quarterback does a lot of the same things Cassel can do.

"They have a similar skill set. They both can throw the ball, they're both experienced," he said. "Matt has more game time, and that's the biggest difference. Actual time in the game." There's an even more precipitous drop-off in experience if Stanzi is forced to play.

The Chiefs' fifth-round draft pick last season spent the majority of the year on the practice squad, and never appeared to be a viable option when Cassel missed the end of the season with an injury to his throwing hand. Tyler Palko and, later on, Kyle Orton took those snaps.

Stanzi was just 9 of 18 for 91 yards with an interception during the preseason. "I'm confident he knows the system. He's been with us," Crennel said. "He's as ready as he's going to be, and that's not going to change in a few days."