KANSAS CITY, Mo. -- Quarterback Matt Cassel, a key reason the Kansas City Chiefs have emerged as contenders and lead the AFC West late in the season, underwent an appendectomy on Wednesday.

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.