Ka'aihue hit a go-ahead double in the seventh inning to help Chen and the Royals beat the Cleveland Indians 6-2 Sunday.
Manager Ned Yost got his 500th career win in the majors. The Royals are 43-52 since Yost replaced Trey Hillman on May 14.
"Chen was on the attack, getting after it," Yost said. "He stayed ahead of hitters, changed speeds and threw them off-balance. He's capable of doing that whenever he goes out there."
The 33-year-old left-hander, with his 10th major league team since 1998, allowed two runs and six hits over six innings.
"I was mixing different arm angles and getting ahead in the count," said Chen (9-7). "It all felt good."
The Royals avoided a three-game sweep and pulled two games ahead of last-place Cleveland in the AL Central. Kansas City is 5-17 on the road since July 8.
"Carmona kept throwing the sinker away all day and was pretty tough," Ka'aihue said. "I just looked for him to get one up in the zone. I wanted to be aggressive if he did. Everything came together from there."
Joakim Soria struck out the side in the ninth in a non-save situation. The right-hander has 36 saves in 38 chances and a team-record 28 straight, but had not worked since Wednesday.
Cleveland's final seven batters all struck out.
Jesse Chavez pitched a scoreless seventh, throwing a called third strike past pinch-hitter Michael Brantley with runners on second and third to end it. Blake Wood struck out the side in a perfect eighth.
"The bullpen did a terrific job," Chen said. "Soria is the master. Who knows where we'd be without him."
Carmona lost a career-high fifth straight start as the Indians had a three-game winning streak snapped.
Jason Donald, an injury replacement for Cleveland in the fifth, hit a two-run homer in the sixth to tie it at 2.
"He's a crafty lefty," Donald said of Chen. "It goes to show it doesn't matter how hard you throw. He had a good changeup. He kind of lulls you to sleep."
Donald came on when first baseman Matt LaPorta left with a strained left hip after a flyout to end the fourth. Donald went to second base as Andy Marte moved from third base to replace LaPorta and Luis Valbuena switched from second to third.
"Something cracked or popped," LaPorta said. "We don't think it's anything major. It's a little stiff right now."
He isn't sure if the soreness is related to having arthroscopic surgery on the hip Oct. 14.
"There might be a little bit of correlation between the two," LaPorta said. "It's the same hip, same leg. It could be the fact it might be weaker. We'll see how the next couple days go."
Manager Manny Acta said it's unlikely LaPorta will play Monday in the opener of a three-game home series against the Chicago White Sox.
Chen walked two and struck out four, bouncing back from a bad outing Monday in Detroit in which he gave up five runs, five hits and five walks over 4 1/3 innings in a 12-3 loss.
Pena's bases-loaded sacrifice fly put the Royals ahead in the fourth. Maier followed with another sacrifice fly for a 2-0 lead.
Carmona gave up three runs and six hits over 6 2/3 innings. The All-Star right-hander is 0-5 with a 5.97 ERA in five starts since beating Boston 6-5 on Aug. 2.
Carmona has a decision in 19 consecutive starts, going 7-12 since lasting 6 2/3 innings May 17 in an 11-inning loss at Tampa Bay.
Indians great Bob Feller, 91, watched from his customary seat in the press box. The third-oldest living Hall of Famer had missed five games while having treatment for leukemia. "I feel better and the doctors say it is treatable," Feller said. ... The Royals activated RHP Robinson Tejeda and put RHP Kanekoa Texeira on the 15-day disabled list, retroactive to Saturday, with a strained right elbow. Tejeda had missed 27 games with a strained right biceps. ... Pena was hit in the left arm by the bat of Indians OF Shelley Duncan on a follow-through swing in the fourth, but kept playing. "Those look worse than they feel," said Yost, a former catcher.