It was a bad game all around for the D-backs, who fanned 16 times at the plate and were not sharp defensively.
"Just a game to forget," D-backs manager Kirk Gibson said.
The D-backs will try to do that and come out fresh in Sunday's finale as they attempt to win the series and finish their six-game road trip with a .500 record. The D-backs are 13-1 in their last 14 series-ending games.
Miley's outing was not the type of performance the D-backs have become accustomed to seeing.
Coming into the game, Miley had allowed just four earned runs in his previous four starts, spanning 30 2/3 innings. When he walked off the mound with two outs in the fourth, the Brewers had already scored eight runs.
"Obviously, today I don't think he had his best stuff," D-backs catcher Miguel Montero said. "Eventually he's going to have games like that. He's a human being. It was not a good outing. He was still battling and he was still competing. Unfortunately, things didn't work out the way he wanted."
The Brewers jumped on top, 1-0, in the second inning and then in the third Ryan Braun increased their lead to 2-0 with a solo homer, the first of two on the night for him.
The fourth inning would prove to be the backbreaker for the D-backs and Miley.
Corey Hart and Rickie Weeks led off the inning with singles and Cody Ransom followed with a home run to left to give Milwaukee a 5-0 lead.
Ransom played for the D-backs earlier this year before the team decided to designate him for assignment and he was claimed on waivers by the Brewers.
Miley (9-4) was almost able to get out of the fourth as he managed to get two outs before allowing an RBI triple to Carlos Gomez. Braun then sent him to the showers with a two-run homer to push the Brewers lead to 8-0.
"I didn't execute pitches at all," Miley said. "I got behind a lot. When I was in a 1-2, putaway count, I threw pitches down the middle of the plate. I just wasn't executing. I got away with some stuff early, but the whole game I was living in the middle of the plate. At this level, you can't live up there. You've got to make quality pitches. I wasn't able to do that tonight and it got us in a hole early."
The D-backs could do little with Brewers rookie starter Mike Fiers, who was making his seventh big league appearance and sixth start.
They had some opportunities early, drawing a pair of walks in the first inning and collecting two singles in the second, but all four runners were stranded.
"It wasn't really them, it was me not hitting my spots," Fiers said. "Early on, I was trying to get in a groove and I couldn't."
When he did, the right-hander was impressive. He allowed just two hits in six shutout innings while walking three and striking out a career-high 10.
The D-backs' first run scored on a Gerardo Parra groundout in the eighth inning. Montero then hit a solo homer, his eighth, in the ninth.
"He was a little deceptive and he got out of the first inning and settled down," Montero said of Fiers. "It was a good time for us to score some runs and unfortunately it didn't happen. He threw the ball good. It was just frustrating not being able to put together good swings. We had like 16 strikeouts or something like that. I mean they're good, but I don't think that should happen."