"Today was just not my day," Chen said. "I definitely wanted to bring [Machado] a new hope, a new start. But I didn't do my job, and I just couldn't do it. Tomorrow is another day, and we'll just move forward."
Although the loss snapped a five-game winning streak, the O's (60-52) remain tied with the Tigers for the American League's second Wild-Card spot.
Thursday also marked the end of a strong five-game stretch for Chen, who entered 3-1 with a 2.82 ERA since the All-Star break before giving up as many runs as he had allowed in those five starts combined.
Showalter said the Royals "ambushed" Chen early, scoring four runs on his first eight pitches. Alex Gordon led off the game with a homer, and Billy Butler added a three-run blast following back-to-back singles by Alcides Escobar and Lorenzo Cain.
"[Chen] came out throwing fastball and missed out over the plate some," said Butler, who finished a single shy of the cycle. "It's the Major Leagues, you're going to run into a rough start every now and then."
"He's human," Showalter said of Chen. "He's performed at a very high level for us, and I think it's more them than it was him."
The first out Chen recorded was a liner by Salvador Perez that bounced off his left upper calf to second baseman Robert Andino, who threw to first for the out.
Showalter, team trainers and pitching coach Rick Adair came out to check on Chen, but ultimately left him in the game despite his pain and early struggles.
"I was impressed with his toughness there to keep going," Showalter said. "He got a pretty good shot there."
But Chen, who said the injury was "not a big deal," continued to pitch, and Kansas City continued to score.
Butler spurred the offense again in the third, hitting the first of three doubles for the Royals in the inning. Mike Moustakas scored Butler with a double down the first-base line, and Jeff Francoeur plated Moustakas, when he sliced another into the right-field corner.
"I didn't have really good command today," Chen said. "My fastball, I feel like, was really straight today. There was no life on that."
Butler scored his third run in the sixth to stretch the Royals' lead to 7-0. After Butler hit a one-out triple on a ball that got by a diving Nick Markakis in right, Salvador Perez plated him with a sacrifice fly.
Moustakas followed with a single -- the career-high ninth hit against Chen -- to end the left-hander's night.
"I know his competitive nature, and he doesn't like getting hit around," Showalter said. "I'm not saying it's good for him because he doesn't assume anything, and we don't assume how hard it is to do what he's been doing for us, and he'll get back on the horse again."
Chen's struggles proved too much for the offense to overcome, even with two hits from Machado, the O's top prospect.
Machado hit a triple to the right-center field gap for his first Major League hit to lead off the fifth, scoring on Markakis' sacrifice fly.
"He's got that look on his face," Markakis said of Machado. "That's something you don't see too often in young guys like that."
Machado also notched an infield single in the seventh, giving him two of the six hits the O's were able to muster against Royals starter Will Smith, who allowed only two runs in seven innings.
"He handled it well," catcher Matt Wieters said of Machado on his debut. "He made some nice plays at third. His swing, he kept it calm. He stayed nice and relaxed like he had been all year. You can tell that he can handle it."
Wieters had a strong game in his own right, finishing 2-for-3 and driving in the second O's run with a solo homer in the sixth.
In his past three games, Wieters is 7-for-14 with three doubles, three home runs and eight RBIs.
"Just trying to put the good part of the bat on the ball," Wieters said. "I've been fortunate to get some pitches to hit and not miss them."