Diamond, who is appealing a six-game suspension handed out on Friday, tied a season high by surrendering five runs on seven hits and a walk over seven innings.
"I definitely felt pretty energetic out there," said Diamond, who fell to 10-6 with a 3.21 ERA. "Early on I felt like I was locating a lot of my pitches. Curveball wasn't exactly what I hoped it would be. But fastball was working, and changeup as well."
But it wasn't all on Diamond's shoulders, as the offense failed to produce yet again against the Mariners. The Twins lost their seventh straight game to Seattle and have scored just 10 runs in those setbacks.
"I don't know what to say -- we're just floating around offensively," Gardenhire said. "We've got guys who can hit, but we're not putting it together. It's kind of frustrating for everybody involved. We seem to hit some hard but right at them. But we didn't hit too many hard tonight."
Diamond cruised through the first two frames before giving up a two-run double to Kyle Seager with two outs in the third after Brendan Ryan doubled and Franklin Gutierrez walked.
Diamond was hurt by the long ball in the fifth, when Dustin Ackley launched a three-run homer to score Trayvon Robinson and Ryan, who both singled.
"It was on 0-1 fastball, just middle-in a little bit," Ackley said. "Or it might have been right down the middle. It had to be for me to get that one out right now. I knew I hit it well, but out of the box I took off [thinking] like double, triple, inside-the-parker, because you never know with the walls out there, which way the ball is going to kick. I didn't know if it was out, but I knew I hit it pretty well."
Diamond was outdueled by right-hander Hisashi Iwakuma, who held the Twins hitless until Jamey Carroll singled with one out in the fifth inning. It was the only hit he gave up in six innings, but he walked four.
"He was effectively wild," Carroll said. "He was using a lot of his offspeed stuff and his split more. We just couldn't do anything with it."
Iwakuma allowed only an unearned run in the third on a sacrifice fly from Ben Revere after Carroll walked and advanced to second on a wild pitch and third on a passed ball from catcher Miguel Olivo.
"Iwakuma did it the hard way," Mariners manager Eric Wedge said. "He was erratic, didn't play up the same command he normally does, but he didn't give in to it. A lot of leadoff runners, but he controlled the damage."
Four Mariners relievers combined to close it out -- with Charlie Furbush giving up an RBI double to Ryan Doumit in the ninth -- to saddle the Twins with their 16th loss over their last 19 games. Minnesota is now a season-worst 25 games under .500.
"It's been a struggle," Carroll said. "We didn't really put up any threats whatsoever. So yeah, it's frustrating. Obviously, it's not what we want to do and where we wanna be, especially this time of the year when we felt like we were starting to play better. You can go into the last month of the season and continue to build. But it feels like we've taken a step backwards."