The switch-hitting Walker batted .295 in 24 games with Tennessee and spent the bulk of his season at Triple-A Iowa, where he hit .200 in 54 games.
"I was stretching a single into a double, slid into second, hit the bag and twisted it," Walker explained. "My lead leg hit the bag and I kept on going. I'm out for the year. It's just one of those injuries that take awhile to heal.
"Right now, it's just a lot of pull work. I can't put full, 100 percent pressure on my ankle, so I've been doing a lot of pull work, some balancing drills and a couple of strengthening exercises," he said.
Walker entered the season a career .275 hitter in five minor league seasons and had made three consecutive All-Star teams at Class A Lansing in 2004, Class High A Daytona in '05, and Double-A West Tennessee in 2006.
He led his league in stolen bases each of those years.
"It's tough," Walker said. "I'm used to being out there and now I can't play. There's nothing you can really do now but just get healthy."
The Cubs selected the 27-year-old Walker in the sixth round of the 2002 draft from Georgia Southern.