The 32-year-old former England forward, who has only played 54 minutes for the Premier League side since joining at the start of the season, left Manchester United at the end of last term after injury had again threatened to wreck his career.
"Michael is so nervous about it being a long-term injury," Stoke boss Tony Pulis told the Daily Mail newspaper.
"It's more to do with his posture than anything else. That affects the hamstring. It's all about the alignment of his back and spine, making sure everything's in place."
"We believe there are a lot of problems that you get in muscular injuries and the source is your spine not being in line. But we knew this might be a stop-start campaign for him, especially at the beginning," Pulis added.