Diamond was pelted for 10 hits -- matching a season high -- while allowing four runs in Minnesota's 9-1 loss to Kansas City in the series opener before 28,993 at Target Field.
Diamond -- making his first start since Sept. 4 -- also issued a season-high three walks en route to his third straight home loss after starting the season 6-1 on his home field.
"I just feel a little inconsistent right now," Diamond said. "I feel like my stuff is getting where it needs to be, but my fastball location is off. My command's just a little off. It's just something you have to battle with. All the time off and getting ready and prepared -- I have to be able to walk in and focus a little better than I did tonight."
Despite his struggles, Diamond prevented the night from becoming worse as he stranded seven Royals runners over his six innings.
"He got some balls up, didn't bury the pitches like he normally does," Twins manager Ron Gardenhire said. "But I also thought he hung in there pretty good. The game could have gotten really away from him. He got out of some big situations."
The Twins' defense didn't help Diamond much in the second inning, when the Royals grabbed a 3-0 lead.
Diamond surrendered singles to three of the first four batters -- one of which was an RBI single by Jeff Francoeur. But the southpaw rebounded to strike out Johnny Giavotella for the second out. Lorenzo Cain then lifted a fly ball to left field but Josh Willingham -- playing his first game in the outfield since Sept. 1 -- lost track of the ball and it fell in for a triple that brought home two runs.
Willingham said the lights weren't an issue on the play, it was the time of day.
"It's frustrating when you can't see the ball," he said. "You can't catch it if you can't see it. You'd like for that stuff to not happen."
Meanwhile, rookie Will Smith went seven scoreless innings for his first win against the Twins after losing two previous starts against them this season. Minnesota was held to one run or fewer for the 24th time this year.
"He was effective," Jamey Carroll said of Smith. "We've gotten to him before. Tonight we just didn't have much to offer for him."
Smith struck out a season-high seven batters but wasn't entirely unhittable as Minnesota scattered seven hits in his seven innings.
The Twins stranded one runner in six of those seven frames, including Ben Revere at third base in what initially appeared to be a promising first inning. Smith started the game by surrendering back-to-back singles to Revere and Carroll, but he sandwiched Carroll's caught stealing at second with strikeouts of Willingham and Morneau.
Minnesota also started the fourth favorably, as Willingham led off with a single and Morneau drew a walk. Then Smith induced a double-play ball from Ryan Doumit and Trevor Plouffe struck out.
"He didn't walk too many and pitched ahead in the count. He pitched to the corners," Willingham said. "[He] just did all the good things you need to do to be effective. He didn't leave a whole lot of stuff over the middle."
Minnesota got on the board in the bottom of the eighth with a walk and two hits, including an RBI single by Doumit. But that was all, as Plouffe struck out to end the inning with runners at second and third. And by then, the Royals had already distanced themselves with four more runs in the inning's top half.
Alcides Escobar's triple in the eighth -- that plated two runs -- was the third of the night for the Royals.
"Those four tack-on runs in the eighth were huge," Kansas City manager Ned Yost said. "It put the game away for us, but up to that point, they're still just a couple of swings and hits away from tying the ballgame up, [and it] can happen quick. But Will did a nice job of making sure that didn't happen."
Kansas City scored another run against Diamond in the fourth with Salvador Perez's sacrifice fly.
Perez was also a factor in shutting down the Twins' running game after they entered with 16 steals in their previous 11 games. In addition to gunning down Carroll in the first, he also nailed Darin Mastroianni at third in the fifth. Mastroianni had previously stolen 19 bases on 21 attempts.
"[Perez] can throw from all angles," Gardenhire said. "The submariner he threw (to third to get Mastroianni) ... it's pretty impressive to be able to throw that ball sidearm down to third like he did. He's an impressive young man."