The surgery, to be performed by team physician Steve Joyce, is expected to keep Sweeney out of the lineup between two and six weeks, depending on the severity of the injury.
Sweeney has been on the disabled list since June 18 because of inflammation in the right knee.
"We tried the non-surgical approach with the cortisone shots and the gel shots, but those didn't work," Sweeney said on Wednesday afternoon. "The first time I tried running on it since going on the DL was on Monday and on Monday night after the game, I couldn't really walk and on Tuesday I couldn't really walk.
"It's been killing me."
Ironically, the back miseries which have hounded Sweeney for years have been absent this season.
"My back feels great," he said, "but the knee doesn't feel the same."
Sweeney, 33, isn't certain what precisely will be involved in the surgery.
"I think they're just going to go in and do an arthroscopic clean-up," he said. "I think there are some loose fragments floating around in there and he'll maybe clean off the underside of the kneecap, maybe smooth out the cartilage under the kneecap."
Earlier, he had been diagnosed as suffering from chondromalacia, defined as a roughening of the articular cartilage, best known for being on the underside of the patella (kneecap).
Sweeney, the Royals' designated hitter, is batting .245 with seven home runs and 31 RBIs in 57 games. He was the club's regular No. 4 hitter.
The surgery probably means that rookie Billy Butler, who has been filling in for Sweeney, will remain with the Royals instead of being sent back to Triple-A Omaha.
After talking a few minutes with Butler in the clubhouse on Wednesday, Sweeney told him: "Go out and drive in six more runs."
Butler had six RBIs in Tuesday night's 17-3 rout of the Seattle Mariners.
"The kid can hit, he has a passion for the game, he knows the game," manager Buddy Bell said. "He's a good fit for us."
Given his nature, Sweeney was smiling and upbeat as he discussed the coming surgery.
"It'll be OK. I don't want to be around here mule-lipping and saying 'Poor me.' I've been very blessed. It is my first surgery in 17 years of playing pro ball. I want to be the same guy hitting .300 as I am hitting .250 getting knee surgery," he said.
"I want to let my teammates know that you can still have joy in the storms of life. And right now it is a storm of life, but I'm still going to have joy and I'm going to do my part to cheer on the guys and be an encouragement to them."
Dick Kaegel is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.