The 27-year-old Australia international sat out the end of last term due to a more serious metatarsal problem, but recovered to feature in the Asia Cup for his country.
Cahill told the club's official website: "I am just devastated. I felt really sharp in the first 45 minutes against Bremen.
"I got a few touches of the ball and had a few chances and possibly should have scored.
"It is frustrating as I was so excited to come back from the Asia Cup. I worked so hard in the summer, not only to get fit for Australia, but for Everton.
"It's definitely not as serious as before. I was trying to tell myself nothing was wrong with me after the game, but the next morning I felt it.
"It is another freak injury. I've seen the surgeons and had scans and I am waiting for some more reports back. I'm just devastated because I've not been injured this much in my life."