The Cubs placed Zambrano on the disqualified list last month after the pitcher had yet another bizarre incident in Atlanta.
Zambrano was ejected from an August 12 start against the Braves for throwing at Chipper Jones. The hurler had already been tagged for five homers in just 4 1/3 innings when he twice threw at the veteran slugger and was subsequently given the boot.
After leaving the dugout for the clubhouse, Zambrano cleaned out his locker at Turner Field and said he was retiring. The Cubs put him on the disqualified list the following day.
A grievance was quickly filed with the union and the case has not yet been resolved. However, the Cubs said once the 30-day period ends Zambrano would have "insufficient preparation time... to pitch again this season."
The Cubs will pay Zambrano for the remaining 2 1/2 weeks of the season and will continue the grievance process in the offseason.
Zambrano has had numerous issues during an often-stormy 11-year tenure in Chicago, including a suspension last year after a heated dugout exchange with then-teammate Derrek Lee.
The 30-year-old hurler owns a record of 9-7 with a 4.82 earned run average in 24 starts this season.