How long the Chiefs might be without Cassel, the AFC offensive player of the month, was uncertain.
They said the procedure was a success and they expected Cassel to "return to work this week." They did not say whether he would be ready to play on Sunday when the Chiefs (8-4) travel to San Diego for a showdown with the Chargers, who trail Kansas City by two games in the division.
There won't be an official injury designation until Friday, but Cassel is being viewed as questionable for Sunday's AFC West showdown at San Diego, a league source told ESPN NFL Insider Adam Schefter. The team believes there's a 50 percent chance of him playing against the Chargers, the source said.
Taking most of the practice snaps on Wednesday was Brodie Croyle, who is 0-9 as an NFL starter and has not appeared in a game all year.
While the Chiefs released no details of the 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 on Tuesday morning and then appeared at another event on Tuesday night. The Chiefs did not say if he underwent emergency surgery, which might suggest a more invasive procedure.
The Chiefs also signed quarterback Tyler Palko to the 53-man roster from the practice squad.
"It's actually a coincidence. It really is," said coach Todd Haley, who insisted during his news conference on saying only that Cassel had "an illness."
Haley declined to speculate whether Cassel 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."
In his second season as the Chiefs' full-time starter, Cassel has been thriving under the tutelage of Haley and first-year offensive coordinator Charlie Weis, hitting 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, going 90-for-144 for 1,111 yards and 12 TDs with only one interception.
Croyle, a third-round pick out of Alabama in 2006, was projected as the Chiefs' foundation quarterback before Herm Edwards was fired as head coach after the 2008 season. Injury prone, he immediately dropped to the second team when new general manager Scott Pioli traded for Cassel before the 2009 season. Croyle has not started since the 2009 season opener, when Cassel was hurt.
"The good thing is I think there's a bunch of similarities," Haley said of Croyle and Cassel. "I think they can both be viewed as dropback quarterbacks. They both have enough mobility to get themselves out of trouble. Arm strength, those things, there's a lot of similarities between the two."
Croyle said this week will be no different from any other for him, other than the fact he'll be getting 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."
Haley said Croyle should still be prepared.
"It's our job to have each and every player ready to go," Haley said. "We constantly remind players you don't know when your number is going to be called. Before every game, we talk to our quarterbacks who aren't necessarily the starters about what are their favorite plays in the game plan, what do they like, so you'll have a game plan for each guy."
"If it is a situation where [Croyle] plays," Haley added, "I know the guys will kind of embrace that opportunity to help him have his best day."
Croyle has seen action in 16 regular-season games, going 173-for-300 for 1,631 yards, eight touchdowns and eight interceptions.
Information from The Associated Press contributed to this report.