Berg had a perfectly good excuse for his struggles that day, though: he was sick as a pooch.
"He had a bad case of the flu that he didn't tell us about," recalled Dennis Lewallyn, Berg's pitching coach at Double-A Tennessee.
"He came out of the game and then he goes and starts throwing up in the clubhouse. You respect him for trying to tough it out, but at the same time, that's something we need to know from a coaches standpoint. That right away kind of put him behind the 8-ball," Lewallyn said.
But Berg has showed signs of climbing out of his early season rut. Although he has just one victory to his show in eight outings thus far, the 22-year-old had a 2.95 ERA in his last four appearances from the Smokies' starting rotation prior to his scheduled start Wednesday against Jacksonville.
He strung together back-to-back seven-inning starts on May 2 and May 7, respectively, in which he surrendered just two runs in each outing.
Most recently, Berg gave up a season-high 11 hits in 5 1/3 innings last Thursday at Mississippi but did not allow a walk for the first time this year.
Having walked 20 in 33 1/3 innings combined – an average of over five per nine innings – this season, Berg knows that the first step toward long-term success is fewer free passes.
"Working ahead in the count is the main thing," Berg agreed. "The odds go so much more in your favor when you do that."
Had walks been as frequent in his eighth start of the season six days ago as they have been at times this year, Berg may have yielded fewer hits but would likely have exited sooner, and with more runs allowed.
Instead, "Eight of the 11 hits were on the ground, so they found some holes," Smokies manager Pat Listach said. "He had a good sinker working."
Most of what Berg does is with the sinker. He was consistently in the low 90s with his fastball against Mississippi hitters, topping out at 94 mph.
To complement his sinker, Berg features a changeup, slider and two seldom-used pitches – a four-seam fastball and straight curveball – in his repertoire.
Berg began using the curveball more and the slider less often during his first full season in the Cubs' farm system a year ago at Class-A Daytona.
He has gone back to the slider since making the jump to Double-A.
"It seemed to work out OK except for the difference in my arm slot," Berg said of the curveball. "Hitters pick that up, especially at this level. So I went back to the slider just for the sake of hitters (not) being able to see it."
The slider is Berg's self-titled favorite pitch, but he hasn't shelved his less used pitches entirely.
"I want to keep my curveball in my back pocket just as a get-me-over pitch," he explained. "I want it to be in the back of hitters' minds that: ‘Oh, he has another pitch.' My four-seamer, I normally only use it when I'm pitching out or want to climb the ladder to get a guy at the letters."
With a few good starts under his belt recently, now it's all a matter of consistency for the Wisconsin native.
"When he gets that consistency back, I look for good things from him," said Lewallyn. "He's worked very hard on his (slider) and changeup. He has a big league sinker, but it's inconsistent. What we need from Justin now is for him to put together about five or six starts without any interruption."