Zambrano will remain on the list for 30 days and is not permitted to have any contact with the team. Reports indicated that the players' union could take action to appeal the decision.
Zambrano, of course, is known for his fiery and sometimes gruff demeanor. But teammates and Cubs manager Mike Quade seemed confused when Zambrano cleaned out his locker after Friday's game and said he was retiring.
"He's retiring?" Cubs outfielder Marlon Byrd said. "That means he's had enough then. People retire for a certain reason. That means they're done with the game. If that's the case, I wish him well."
In a news release, the Cubs said Major League Baseball and the players' union would use the 30 days to "discuss recent statements attributed to Zambrano regarding his baseball future."
The 30-year-old right-hander tied the Cubs record by giving up five homers against the Braves, who won the game 10-4.
Zambrano allowed eight runs on eight hits over 4 1/3 innings and is 9-7 this season with a 4.82 earned run average in 24 starts.