Diamond gave up four runs in the sixth on three straight two-out doubles and was simply outdueled by Tigers right-hander Rick Porcello, who tossed seven scoreless innings to give the Twins their second straight loss.
"It was a tough night," Twins manager Ron Gardenhire said. "We didn't do much offensively. Porcello was tough. His ball was diving all over the place and he had a nice breaking ball with his fastball coming back over the plate. Diamond matched him for a long time there, but then walked a guy in the sixth and got us in that awful situation where he had to walk a guy and didn't work out too well with three doubles in a row."
Diamond cruised through five scoreless innings before running into trouble in the sixth. It started when he walked Austin Jackson with one out, but was able to get Torii Hunter to ground back to the mound for the inning's second out.
The Twins opted to walk Miguel Cabrera with first base open to get to Prince Fielder, but Fielder made them pay with a two-run double off the wall in right field. It came on a 2-0 curveball from Diamond that caught too much of the outer half of the plate.
"Historically, I've been pretty successful against Fielder," said Diamond, who had limited Fielder to just one hit in 14 career at-bats heading into the game. "I thought it was a good pitch, but I think I earlier in the game I had a chance to buzz him and get him off the plate and I didn't take advantage of it and it cost me in that at-bat. It kind of let the floodgates open in the sixth."
Victor Martinez followed with an RBI double to left field before Jhonny Peralta laced an RBI double of his own to knock Diamond from the game.
It went from a strong outing to a rough one in a hurry, as Diamond ended up being charged with four runs on five hits and two walks over 5 2/3 innings to see his ERA rise to 5.29 on the season.
"Just the way it all played out tonight was really frustrating," Diamond said. "I can't let that inning blow up like it did. The three straight doubles doesn't help. It's just frustrating it all happened in one inning."
Diamond was outpitched by Porcello, who surrendered just three hits over seven scoreless frames. Porcello struck out five and didn't walk a batter.
"He was terrific," Tigers manager Jim Leyland said. "He kept the ball on the ground. A good sinker, but the stuff that really set his sinker up lately is his secondary stuff. They're not just sitting on a fastball anymore. That's been a huge difference for him in my opinion."
The Twins didn't have many chances against Porcello. Clete Thomas was the first to get a hit with a leadoff single in the third, but was thrown out by catcher Alex Avila while trying to steal second to end the inning.
Ryan Doumit was the next to single with two outs in the fourth, and reached second on a throwing error from second baseman Omar Infante. But he was stranded there, as Josh Willingham struck out for the inning's final out.
Oswaldo Arcia had the lone extra-base hit for Minnesota with a one-out ground-rule double over the head of Jackson in center in the fifth. But Thomas flied out to center and Brian Dozier grounded back to Porcello to leave Arcia at second.
The Twins threatened with one out in the eighth, as Thomas walked and Dozier reached on catcher's interference with Drew Smyly on the mound, but Joaquin Benoit came in and struck out Pedro Florimon and got pinch-hitter Chris Parmelee to ground to second to end the potential rally.
"We didn't really answer with any runs or any opportunities really," Gardenhire said. "It wasn't a good night for us, mostly because of their pitching."