Garcia came undone after Betemit's second-inning homer. Chen recovered from a shaky start, retiring Albert Pujols with two on and two out, helping the Royals go on to beat the Cardinals 10-3 Sunday.
Garcia, who entered with the majors' second-best ERA, had looked Chen's opposite until Betemit took him deep.
"It's amazing," Chen said. "I'm struggling the first two innings. He's breezing along. Then he's out of the game and I'm still pitching. I feel very lucky. I feel very grateful."
Jason Kendall had a pair of two-run doubles and Jose Guillen stretched his hitting streak to 21 games as the Royals took two out of three from their in-state rivals. Last year, the Cardinals went 5-1 against Kansas City and swept all three in Kauffman Stadium.
Betemit's home run also snapped a 69-inning homerless drought for Kansas City.
"To be able to take a good swing on a ball and put the ball in the seats is big for us," manager Ned Yost said. "It changed the whole flow of the game. We have two outs, and end up coaxing two walks and Wilson puts a ball in the stands opposite field. It just changed the whole momentum and the whole complexion of the game."
To Tony La Russa, the turning point was getting only one run off the struggling Chen after the first three batters reached in the first.
"That's a big potential momentum swinger. They had a guy warming up in the second inning and Chen just battled his tail off and ends up getting the win," said the Cardinals manager. "It could have changed dramatically. We missed. Lot of that was [Chen]. He's got a lot of weapons out there. He's very crafty."
Chen (4-2) went five innings and gave up two runs on two hits, including Albert Pujols' solo homer in the fifth. But that was after Pujols missed his chance to break the game open three innings before, flying out to center.
"I wish it would have gone out, but it didn't," Pujols said. "If I would have got a big hit in the second inning, I think things would have been different. I didn't come through for my ball club. As a hitter, it's frustrating, but at the same time it's not automatic. I get that situation, more than 50 percent I come through with a base hit."
Garcia (7-4) lasted only two innings plus three batters and was charged with five runs on four hits and two walks. It was his shortest outing and the most runs he's allowed this season. His ERA climbed from 1.79, second-best in the majors, to 2.27.
"It's just a bad day. No excuses, no nothing," Garcia said. "It's just one of those days where it's going to happen. It's just one of those days where the stuff is not there."
Guillen had two singles, his first glanced off the glove of shortstop Brendan Ryan and drove in a run. His 21-game streak tied Rey Sanchez for the longest run by a Royals player, and also is tied with Josh Hamilton's for the most in the big league this season.
During Guillen's streak, 27 of his 31 hits have been singles. He's also had two doubles, a triple, one homer and 13 RBIs.
In the ninth, Colby Rasmus hit a 429-foot solo homer for St. Louis.
Besides giving Chen a boost, Betemit's drive broke the Royals' longest homerless streak since a 71-inning drought in 2007.
"It made me want to go out there and challenge those guys," said Chen.
Kendall had two-run doubles off Adam Ottavino in the fourth inning and the eighth. David DeJesus had a pair of RBI singles. ... Before Betemit's second-inning shot, the Royals' last home run had come on June 18 at Atlanta, by Billy Butler. ... Pujols entered the three-game series batting .400 in 27 games in his home town, with 12 home runs and 36 RBIs. He was 2 for 10 for this series with a single and a solo homer. ... Betemit's home run gave him three homers and three doubles in 18 at-bats.