The Angels exercised Santana's $13 million option for next season before trading the Dominican veteran and cash to the Royals.
"We're proud to have him, and he certainly fits with what we're doing in 2013," Royals general manager Dayton Moore said.
Santana has been a fixture in Los Angeles' rotation since 2005, going 96-80 with a 4.33 ERA while starting at least 23 games in eight consecutive seasons. He has spent his entire career with the Angels, who signed him as a free agent in 2000 and nurtured his development into a solid starter of 96 games over the past three years.
But Santana struggled for much of last season, going 9-13 with a 5.16 ERA while giving up a major league-worst 39 homers despite significant improvements over the final two months.
"I have to realize that this is a business," Santana said, citing Kauffman Stadium as one of his favorites in the majors. "They trade me, and I just have to accept it and move on. It's going to be hard because I've known my teammates for a long time, but now I'm going to have new teammates. I can't wait to meet them in person and hang out."
Santana's woes and Dan Haren's similarly slow start were major factors in the Angels' midseason struggles, forcing them to make a trade for Zack Greinke and ultimately keeping them out of the playoffs for the third straight season.
"I don't have any physical problems," Santana said. "Everything was good. I just have bad luck. I was pitching good, and I didn't have the opportunity to win a lot of games."
With rising starter Garrett Richards already slated for the Angels' rotation in 2013 alongside ace Jered Weaver, C.J. Wilson and hopefully Greinke, Santana has long realized he might be headed out of Orange County.
"Ervin had a lot of really great moments with the Angels, and has always been an Angel," said Angels GM Jerry Dipoto, who just finished his first season with the club. "This is just the start of something new for Ervin, and we wish him well."
In their annual hunt for starting pitching, the Royals made a similar deal last November for Jonathan Sanchez, giving up Melky Cabrera for the San Francisco starter who was expected to anchor their rotation. But Sanchez was a disastrous flameout, going 1-6 with a 7.76 ERA in just 12 starts before getting designated for assignment and eventually traded to Colorado.
Moore wasn't shy about dealing for Santana with just a year left on his contract, saying he hadn't discussed a long-term extension with the right-hander.
"We were able to scout Ervin during the entire year, specifically the second half," Moore said. "His August and September were really good. From a statistical standpoint, he was very good."
Moore said the Royals are paying "the majority" of Santana's $13 million deal. Santana had a $1 million buyout.
Santana, who will turn 30 in December, has shown moments of brilliance in his career, which includes three seasons with at least 16 victories and an All-Star selection in 2008. He threw a no-hitter in 2011 and a one-hitter last season, yet confounded the Angels with his inconsistencies in 2012.
"Ervin struggled out of the chute," Dipoto said. "I think his struggles were more relative to his command than his physical stuff. Once he determined that his issue was command, he throttled back his fastball a little bit and tried to gain command of his slider and pitched very well for us. To say Ervin's season was a failure would be unfair. Ervin's season was very uneven, and he took it down the stretch and gave us an opportunity to be in the race until the final week of the season."
The Angels are more interested in developing Richards and attempting to re-sign Greinke, who went 6-2 with a 3.53 ERA during his impressive but brief stint with Los Angeles.
The Angels haven't announced whether they'll pick up Haren's $15.5 million option for next season before Friday's deadline, and Dipoto gave no hints Wednesday. The club is thought to be shopping Haren, who had a career-worst 4.33 ERA and spent time on the disabled list for the first time.
Sisk spent last season with Triple-A Omaha, going 3-2 with a 2.54 ERA and eight saves. He's pitching in the Venezuelan winter league and has never pitched in the majors, but Dipoto is eager to see what the 27-year-old can do in spring training.
Los Angeles could use any bullpen help after a mostly dismal season.
"He's always had the deceptive ability to miss bats," Dipoto said. "He's done it at every minor-league level, and hasn't had the opportunity to showcase it in the big leagues. We don't know if that's going to happen. Worst-case scenario is we're building much-needed depth and building toward creating a little bigger fishnet [in the minors]."