Brodie Croyle, who is 0-9 as an NFL starter and hasn't appeared in a game all year, took most of the practice snaps Wednesday. The Chiefs also signed quarterback Tyler Palko to the 53-man roster from the practice squad.
Coach Todd Haley, who insisted on saying during his news conference only that Cassel had "an illness," declined to speculate on whether or not his quarterback will be ready for Sunday's game.
"Again, he'll be on the injury report as illness today," Haley said. "And we'll just take it from there as we do all injuries."
Pittsburgh Steelers quarterback Ben Roethlisberger underwent an appendectomy four days before the 2006 season opener. He returned to play in a Week 2 game -- 15 days after the surgery -- and his passer ratings in that and the following contest -- 38.7 and 30.7 -- were among the six worst of his career.
While the Chiefs didn't release any details of Cassel's operation, the American College of Surgeons website advises patients who have had a minimally invasive appendectomy not to "lift or participate in strenuous activity for three to five days" after the procedure.
Cassel helped present area high school star Bubba Starling an award Tuesday morning, then appeared at another event Tuesday night. The Chiefs didn't say if Cassel underwent emergency surgery, which might suggest a more invasive procedure.
In his second season as the Chiefs' full-time starter, Cassel has thrived under the tutelage of Haley and first-year offensive coordinator Charlie Weis, connecting on 212 of 354 passes for 2,503 yards and 23 touchdowns. He has thrown an NFL-low four interceptions. His current streak of 158 passes without an interception is the third-longest active streak in the league.
Cassel was the AFC's Offensive Player of the Month in November, completing 90 of 144 passes for 1,111 yards and 12 TDs with only one interception.
Croyle, a third-round draft pick out of Alabama in 2006, was projected as the Chiefs' foundation quarterback before Herman Edwards was fired as head coach after the 2008 season. Injury prone, Croyle immediately dropped to the second team when new general manager Scott Pioli traded for Cassel before the 2009 season. Croyle hasn't started since the 2009 season opener, when Cassel was hurt.
Croyle said this week will be no different than any other for him, other than the fact he'll take more practice reps.
"Just got to prepare like you always do," he said. "My number's called, got to be ready to go. You've got to be ready to go on any play."
Croyle has seen action in 16 regular-season games, completing 173 of 300 passes for 1,631 yards and eight touchdowns with eight interceptions.
The Associated Press contributed to this report.