Johnson went 8-6 with a 2.73 ERA in 16 games with Triple-A Pawtucket this season. He last pitched on July 6.
"Some things fell into place because of the break. It wasn't something we did to give him an extended period," said Red Sox manager John Farrell.
Johnson threw in an extended bullpen session on Thursday and will throw another light bullpen session on Sunday.
The team was deciding between starting Johnson on Monday or Tuesday. The starter for Monday's series finale against the Angels has yet to be announced.
• Buchholz will see Dr. James Andrews on Wednesday for a second opinion on his injury.
"It is more peace of mind for him," Farrell said. "It's within his right to do that. We are supportive of it."
Quinn Roberts is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.