Braun connected on Arrieta's ninth pitch of the game with one out in the first after Jean Segura singled, launching an elevated fastball into the juniper bushes in straightaway center.
"I just missed with a heater early in the count to Braun," Arrieta said. "He was able to put a good swing on it. I just wasn't very good today, plain and simple. I didn't do a good enough job. We need more out of our starter and I didn't give us the effort I intended to today."
It was only Arrieta's third loss in his last 16 starts at Wrigley Field.
"They put some good swings on a lot of pitches," Arrieta said of the Brewers. "Too many balls with too much height over the plate. I think they had four bloop hits which led to the two runs in the second. I've got to come out better, that's really it."
Carrie Muskat is a reporter for MLB.com. She writes a blog, Muskat Ramblings, and you can follow her on Twitter @CarrieMuskat. This story was not subject to the approval of Major League Baseball or its clubs.