BOSTON -- Red Sox knuckleballer Tim Wakefield has a broken rib and is expected to spend at least three weeks on the disabled list, manager Terry Francona said Thursday.

Francona said Wakefield doesn't remember how he injured himself. It might have been from sleeping on it wrong. The team put him on the 15-day disabled list retroactive to Tuesday.

"He's actually been pitching with pain in his ribs for about a month or six weeks," Francona said the Red Sox played Texas. "The DL is a definite."

The AL East-leading Red Sox did not announce a starter to take Wakefield's place for Saturday's game at Seattle.

The 39-year-old Wakefield, the longest-tenured member of the Red Sox, is 7-8 with a 4.14 ERA this season. In a 14-year career, he is 151-131 and also has 22 saves.

"The good news is that it shows significant healing," Francona said. "But it's still giving him a lot of pain, especially in the muscles that lead away from that. When you're pitching, I know it was grabbing him every time and I'm sure it was grabbing him a lot."

Wakefield will not go on the road trip that begins in Seattle. Instead, he will rest for three days before starting therapy.

"I think there's a chance we could have him back with us, best-case scenario, three weeks," Francona said. "That's if the stars align. I think four weeks is probably more what were looking at."