Diamond now is 3-0 since being called up for the Twins, who pounded the Milwaukee Brewers 11-3 on Friday night.
"He's not overwhelmed," Gardenhire said. "He went to Triple-A and did his thing. As we said when we sent him down, 'Be the guy that's throwing the best and you'll get your chance.' And here he is."
Diamond (3-0) gave up a run in the first, ending the 14-inning scoreless stretch he put together in two starts since he was called up from the minors. He went on to give up three runs and eight hits in 5 1/3 innings.
"I'm getting to really know our defense, and I'm feeling a lot more comfortable out on the mound," Diamond said. "I trust these guys as much as I can, and I'm just excited to take the mound."
"The big boys in our lineup all had really good nights, along with Denard Span at the top," Gardenhire said.
Ryan Braun hit his 11th home run for the struggling Brewers, who went 1-3 on a short road trip earlier this week. Milwaukee had four errors.
Can Milwaukee still turn it around?
"It can't be too much longer," Brewers manager Ron Roenicke said. "It is a long season. It is not at the point where I still don't think we can't turn this thing around. We've got a long ways to go. I just don't want to have to have the huge winning streaks and monthlong streaks where it is the only way we can get back in it."
Roenicke made a significant lineup change coming into the game, shifting struggling second baseman Rickie Weeks to the No. 6 spot in the order. Coming into the game, Weeks was batting only .156 with 49 strikeouts.
Hart had a single and a double while Aoki added a single and an RBI double -- plus an over-the-shoulder catch in the first. Weeks went 1-for-3 with a walk.
Brewers starter Marco Estrada (0-3) gave up four runs and six hits in five innings, with a walk and five strikeouts.
The Twins took a 2-0 lead in the first when Span led off with a double, then Estrada gave up a pair of deep fly balls -- both going for outs on impressive catches by Aoki and Braun -- before Willingham crushed a pitch deep into the left field stands.
It wasn't the first big moment in Milwaukee for Willingham, who hit two grand slams in back-to-back innings at Miller Park while playing for the Washington Nationals in 2009.
The Brewers got a run back in the bottom half of the inning when Hart scored as Braun grounded into a fielder's choice, ending Diamond's scoreless streak. But with runners on second and third with two outs after a double-steal, Weeks grounded out to end the inning.
Milwaukee tied it up in the third on an RBI double by Aoki, but the Twins threw out Braun on a double-steal attempt that sent Aoki to third -- then third baseman Jamey Carroll threw out Aoki at home on a grounder by Aramis Ramirez, and Jonathan Lucroy flew out to end the inning.
Mauer then delivered a two-run double in the fifth to give the Twins a 4-2 lead.
Brewers reliever Manny Parra loaded the bases with one out in the sixth, and the Twins let Diamond bat instead of pinch hitting for him. Gardenhire said he was concerned about overworking his bullpen.
"I can't take him out of there," Gardenhire said.
After getting his first career hit in the fifth, Diamond struck out.
"He said, 'I think the pitches were high' and I said, 'Be careful, you're getting those pitches on the other side,' " Gardenhire said.
But Span hit a two-run single to put the Twins up 6-2 and chase Parra from the game.
Braun hit a solo homer in the sixth, cutting the lead to 6-3.
"I did hang it, but I think he was right on it," Diamond said. "I've got to make a bit of an adjustment there."
But the Twins added a run in the seventh and another four in the eighth, including one on a bloop single by Span that landed between Weeks and Hart.
Gardenhire said Ryan Doumit was available for pinch-hitting duty Friday despite a sore calf. ... Roenicke said the team was monitoring starter Shaun Marcum, who has soreness near his ribcage. ... Twins RHP Carl Pavano (2-3, 5.14) faces Milwaukee RHP Yovani Gallardo (2-4, 5.04) Saturday.