Berg, 23, struck out 10 batters and walked 11 in 28 1/3 innings, going six innings or more in three of his five starts this season. Opponents batted .264 against Berg.
"His record is not indicative of how he's pitched here," Tennessee pitching coach Dennis Lewallyn said of Berg. "He's throwing the ball well and he has matured a lot as a person."
Lewallyn outlined some of the improvements that Berg has made this season as opposed to a year ago in Berg's first stint above Single A ball.
"One of the things that he was really bad at was holding runners," Lewallyn said. "He's much better at that now and is quicker to the plate with runners on base.
"His focus is better this year," added Lewallyn. "He can get people out in the big leagues if he throws the ball over the plate. He just needs to be consistent."
Berg made 26 starts a season ago at Double-A, going 7-7 with a 4.95 ERA in 140 innings. He walked 69 batters and struck out 69. The Cubs acquired Berg from the New York Yankees in August, 2005, for OF Matt Lawton.
"We've been very happy with him," said Cubs Assistant to the General Manager Gary Hughes. "All he needs is (more) command and to keep making positive steps."
To coincide with the move, Russell was promoted to Tennessee from Class High-A Daytona. In five starts with Daytona, he was 1-1 with a 2.30 ERA in 27 1/3 innings. The southpaw struck out 14 batters and walked seven.
Russell, 22, was the Cubs' 14th-round selection in the 2007 draft.
No other roster moves are known at this time.
InsideTheIvy.com staff writer Maria M. Cornelius contributed to this report.