Medical personnel attended to the fan, delaying the start of the eighth inning Tuesday night. The man had a brace around his neck as he was wheeled off.
The Brewers said the fan was conscious and alert, and was being taken to Froedtert Hospital.
Fans can watch the game from a dining area above the Milwaukee bullpen beyond the wall in left-center. The concourse, which is at least 15 feet above the bullpen, is lined with a metal railing roughly 4 feet high.
Brewers reliever Brandon Kintzler said he thought the fall had killed the man.
"He was on the ground before I started warming up," Kintzler said after the Brewers' 6-4 loss, according to the Milwaukee Journal Sentinel. "I thought he died. We were sitting there and (bullpen catcher) Marcus (Hamel) yelled, 'Oh, gosh!' We saw the net (over the bullpen area) come flying down. I think the net caught him, then he just face-planted. He didn't move for a few minutes. I thought he was dead.
"It's not something you want to see. They're basically giving this guy CPR and I had to warm up. It was tough. It's not the best thing to see at the time.
"It took him awhile to start moving his fingers. We didn't want to touch him for a while because he was motionless. He tried to get up after that and the paramedics told him to stay down. Security called the paramedics and they were there pretty quick."
Information from The Associated Press was used in this report.