Fall League play begins Tuesday. The Solar Sox will face the Phoenix Desert Dogs in the head-to-head opener.
"Grant's not coming," Solar Sox manager Pat Listach said. "He's not here. Harben is starting [Tuesday]."
The Twins drafted Harben in the 15th round (2002). He entered 2006 with a career 28-19 record and an ERA of 3.23 in 79 games, 69 of them starts.
Johnson, the Cubs' first overall selection in the 2004 draft, was 7-5 with a 4.70 ERA at Class-A Daytona this past season. He made 10 starts to open the season, but closed out the year with 21 consecutive relief appearances.
From the bullpen, Johnson was 3-0 with a 4.03 ERA in 38 innings. As a starter, he was 4-5 with a 5.17 mark in 54 innings.
"His velocity is up to where we thought it would be, up to 93 [mph] again," Cubs Farm Director Oneri Fleita said of Johnson in August. "We think he likes the bullpen role and it looks like he fits the role."
Harben will join pitchers Lincoln Holdzkom, Carmen Pignatiello, and Clay Rapada from the Cubs' organization in the Fall League. Position players on hand will be: Jake Fox, Scott Moore, and Eric Patterson.
No other roster moves affecting the Cubs' contingent of prospects were made. The Solar Sox are home to players from the Cubs, Houston Astros, Los Angeles Dodgers, Minnesota Twins and New York Mets organizations.