BOSTON -- Just when it seemed Tim Wakefield had regained his health and was primed to give the Red Sox the All-Star form he provided throughout the first half, the veteran knuckleballer had another setback -- one that will cost him at least his next start.

Wakefield had returned from a near six-week stint on the disabled list to pitch seven terrific innings (six hits, one run) in a no-decision against the White Sox on Wednesday night. In the first two days after that start, Wakefield felt some discomfort in his back. By Saturday, it had completely locked up again.

Jon Lester, who was originally supposed to pitch on Sunday afternoon against the Blue Jays, will instead start in Wakefield's spot on Tuesday night at Tropicana Field for the opener of a key three-game series against the Tampa Bay Rays.

Paul Byrd, the right-hander who came out of retirement on Aug. 5 and signed a Minor League deal with Boston, will get the start Sunday. Byrd has been pitching at Triple-A Pawtucket and last started on Aug. 24. It will be the beginning of Byrd's second tenure with the Red Sox, as the 38-year-old veteran went 4-2 with a 4.78 ERA in eight starts after being acquired from Cleveland last August.

As for Wakefield, he displayed the same type of hobble in the clubhouse on Saturday that he had in Texas in July, when his stint on the disabled list started.

"There has to be some concern just because he's miserable," said Red Sox manager Terry Francona. "And it's the weekend, so there's some things that can be done medically, but not today. Again, I think the best thing they can try to do now is try to get him to relax so the spasming can somewhat maybe die down, which will really help. And then Monday, we'll get him over to see the appropriate people and then go from there."

Francona said it's possible Wakefield will get an epidural to lessen the pain.

"We also need to have him checked by the proper doctors to make sure that's the appropriate step," Francona said. "If it is, he'll get it done, which would give him a lot of relief. That would be the hope."

Lester will have six days of rest entering his start at Tropicana Field.

"First of all, I think this time of the year, any pitcher is going to benefit from a couple of days of additional rest," said Red Sox pitching coach John Farrell. "With Wake's next start somewhat in question, the [uncertain] weather this weekend, we just wanted to take a lot of the variability out of it so Jon can prepare best, as we would do with any other starting pitcher -- not to try to leave them in limbo or hanging in the balance. By making this decision, we're able to have him focus and gear up for Tuesday at Tampa."