Manager Dale Sveum said the right-hander would throw a bullpen session before his first rehab start, which Garza tweeted will be on Wednesday.
The Cubs' original plan was to have Garza make three Minor League starts, but Sveum says things may have changed.
"We'll evaluate all that," he said, "but he hasn't lost any time, so we'll probably stick with three. But it might look a little bit more like four."
Garza is currently on pace to return at the end of May barring any more setbacks. The option to put him on the 60-day disabled list does exist, but Sveum says the team isn't currently considering that.
"It hasn't been brought up," Sveum said. "That wouldn't make any sense unless we absolutely needed a [roster] spot."