KANSAS CITY, Mo. -- Chiefs quarterback Matt Cassel has been ruled out for Sunday's game at Tampa Bay because of 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 spent only 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 has thrown five touchdowns and nine interceptions 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, coincidentally, but wound up getting hurt in the game. He went on injured reserve and eventually was traded to the Denver Broncos. He signed with Kansas City this past offseason.