Diamond turned in his club-leading fifth quality start, tossing seven strong innings to improve to 4-1 with a 1.86 ERA. The left-hander allowed three unearned runs on seven hits while striking out two.
"My confidence is high, but I'm looking more at how comfortable I am," Diamond said. "As that game progressed, I felt more comfortable. When I gave up those two runs [in the fourth], I was a little upset, because I didn't execute as well as I wanted to. But for the most part, I was able to settle down and get back to getting outs."
Diamond was simply able to keep the Indians off-balance and stay in control, as he didn't issue a walk for the second time in six starts.
"He does a really good job of getting his fastball to go downhill," Indians designated hitter Shelley Duncan said. "Also, that low strike, he's got a really good angle to it. It makes it hard to hit. He was locating with his changeup and slider and cutter. Any guy that does that, he's going to do pretty well."
Diamond was backed by a solid showing from the Twins' offense, which got out to an early lead for the second straight game.
Joe Mauer put Minnesota on the board in the first inning with an RBI single off right-hander Justin Masterson, after Denard Span walked and stole second base.
The Twins scored again in the third on Josh Willingham's first RBI double of the afternoon to score Mauer, who walked to lead off the inning.
Minnesota added a run in the fourth with the help of some small-ball tactics, as Brian Dozier and Trevor Plouffe both reached on bunt singles before Alexi Casilla brought home Dozier with a sacrifice fly.
"We kind of manufactured some runs," Twins manager Ron Gardenhire said. "We dropped some bunts down and got around the bases OK."
Dozier made a miscue in the fourth, as Jason Kipnis reached on Dozier's throwing error and scored on a sacrifice fly by Jose Lopez. Shelley Duncan later added an RBI single to bring the Indians within a run.
But Minnesota scored two insurance runs in the seventh against reliever Nick Hagadone, as Willingham hit his second RBI double and scored on a two-out single from Ryan Doumit to make it 5-2.
The Indians threatened in the bottom half, scoring their third run as Matt LaPorta reached on an error by Plouffe and scored on Lou Marson's double. But Diamond was able to strand Marson at third base to limit the damage.
Plouffe atoned for his mistake with a double to lead off the eighth and came around to score on Casilla's second sacrifice fly of the game. After providing the go-ahead homer in the Twins' win on Saturday, Plouffe went 3-for-4 on Sunday.
Relievers Glen Perkins and Matt Capps each threw a scoreless inning to help the Twins to their fifth win in their last six games. Capps also improved to 12-for-13 in save opportunities.
It helped the Twins make up for their two losses to Cleveland at Target Field in mid-May and kept the momentum going into their upcoming three-game set against the Royals.
"They kind of took it to us at our place so it's good to come here and get these wins," said Mauer, who left the game in the ninth with a mild right thumb sprain. "Hopefully we can do the same thing in [Kansas City]."