"He's throwing the ball well and he has matured a lot as a person," Lewallyn said.
The veteran pitching coach, who worked with Berg at Tennessee a season ago, said he offered the hurler one last piece of advice before he left the club for Iowa.
"I told him, ‘Don't change a thing. Throw strikes,'" recalled Lewallyn. "Part of throwing more strikes is concentration."
Berg threw 75 pitches on Wednesday, 45 strikes.
"He was the guy who probably was closest to being ready to compete at the Triple-A level, and that's why they took him," said Lewallyn.
A season ago, Berg was 7-7 with a 4.95 ERA in 140 innings with Tennessee. The Cubs acquired Berg from the New York Yankees in August, 2005, for OF Matt Lawton.