MINNEAPOLIS  — Scott Diamond was the only one in Minnesota’s rotation who pitched well enough last year to keep his place.

Lately, the left-hander has struggled as much as any Twins starter.

Prince Fielder broke open a scoreless game with a two-run double in the sixth inning, the spark that Rick Porcello and the Detroit Tigers used to beat the Twins 4-0 on Friday night.

Victor Martinez and Jhonny Peralta followed Fielder’s soaring drive with RBI doubles of their own, ending the night for Diamond (4-6). Even better for the Tigers was that the four-run cushion kept sputtering closer Jose Valverde out of a pressurized save situation.

“I feel like it’s definitely coming around. I just need to prevent these big innings,” Diamond said. “It’s getting pretty frustrating, and it’s not letting me get later into the game. It’s not helping our bullpen out.”

Porcello (4-3) didn’t need much relief.

The right-hander breezed through seven scoreless innings, allowing just two singles and one double without a walk. He struck out five and gave the Tigers their 19th quality start (of at least six innings and three runs or less) in the last 20 games. Despite his status as the afterthought of this stacked rotation behind Justin Verlander, Max Scherzer, Anibal Sanchez and Doug Fister, Porcello has shined in his last four appearances. He is 2-1 with 30 strikeouts in 27 innings and just 15 hits, five runs and four walks against him during that stretch.

The Twins haven’t had success anywhere near that with their starters.

They have seen marginal improvement with their patchwork rotation recently, with a collective 2.87 ERA over the previous 10 games. But Diamond, the only one who pitched well enough in 2012 to warrant a spot in this year’s group, has been the outlier over the last month.

The left-hander, who was replaced with two outs in the sixth, gave up five hits and two walks with just one strikeout. The six scoreless innings he pitched on June 2 fueled his lone victory of the last seven turns. That is also the only time in that span he finished six innings.

“He was off the center of the plate all night long until that inning when we scored. … But that inning he made a few mistakes, and we jumped on him,” Tigers manager Jim Leyland said.

Diamond matched Porcello frame for frame until he walked Austin Jackson, the leadoff man back in his familiar spot after missing the last month because of a pulled left hamstring, with one out in that decisive sixth. Torii Hunter advanced Jackson with a groundout, and Miguel Cabrera was intentionally walked.

“Historically, I have been pretty successful against Fielder,” Diamond said. “Thought it was a good pitch, but I think earlier in the game I had a chance to buzz him or get him off the plate, but I didn’t take advantage of it. I think it kind of cost me on that at bat. It kind of let the flood gates open.”

Fielder made the Twins pay. With one of those huge upper-cut swings, he smacked a 2-0 curveball high off the towering right-field wall to put the Tigers in front. Cabrera ran through the stop sign by third base coach Tom Brookens, whose arms were still straight up in the air as the big slugger chugged home without a throw.

“Scotty had good numbers against Fielder, but he is so dangerous. He is so strong. He leaves a curveball up, and the guy bangs it off the bottom of the wall out there,” Twins manager Ron Gardenhire said.

Valverde came in for the ninth after Drew Smyly and Joaquin Benoit collaborated on a scoreless eighth. Valverde walked Joe Mauer to start the inning, but Ryan Doumit followed with a double-play ground ball. Josh Willingham was grazed by a pitch, but Justin Morneau struck out looking to end the game.

Hitting has quietly been a bigger problem for the Twins lately. They have just 13 runs in their last seven games.