"I said a couple of times that it was going to get better," Counsell said. "They were better than they were pitching. That's what's happened."
The Brewers still own baseball's worst starters' ERA at 5.43, but that's down over the past 10 games from 6.41. In those 10 games, Brewers starters have combined for a 3.05 ERA.
Guerra has done his part since a promotion from Triple-A Colorado Springs, going 3-0 with a 3.96 ERA in his first four Major League starts. The Brewers have won all four games.
"It feels great," Guerra said via translator Carlos Brizuela. "It feels like everybody is putting a little bit in to help the team win."
Do Guerra's contributions qualify as a surprise?
"He's pitching well. I don't know if I'm shocked," Counsell said. "We had some hopes for him when we claimed him. We saw some things in Colorado Springs that we really liked. He's doing a good job."
Cubs leadoff hitter Dexter Fowler gave the visitors a lead two pitches into the game when he hit a solo home run, and the Cubs scored again in the second inning on Miguel Montero's single. But Guerra settled in to work four consecutive scoreless innings before he surrendered another run on a wild pitch in the seventh.
With 11 strikeouts, Guerra delivered the Brewers' first double-digit strikeout game of the season. Michael Blazek and Tyler Thornburg covered the final two innings for the Brewers' first series win over the Cubs since last May.
"The Brewers pitched really well against us," Cubs manager Joe Maddon said. "Their guy today, Guerra, pitched really, really well. They had a bunch of punchouts. We haven't been swinging and missing like that in a bit. You've got to give them credit."
Guerra's signature pitch is a split-fingered fastball, which is not really a fastball at all but an offspeed offering he added five years ago as a foil to his 94-96 mph fastball. He threw 42 of them in 113 pitches on Thursday, getting swings on half and swings and misses 10 times. Five were swinging strike threes.
"It is rare. You don't see it that much, certainly from starting pitchers," Counsell said. "It's not a pitch you see in player development at all. I think the injury risk, whether it's myth or not, it's there.
"It's a good pitch, and it plays off his other stuff very well. It plays off his fastball very well. He's got enough velocity on his fastball to keep them honest."