ST. PETERSBURG -- Red Sox knuckleballer Tim Wakefield is ready to find out if the cortisone shot that he had in his back on Monday will make him comfortable enough to pitch every fifth day and be a factor down the stretch.

Wakefield played catch before Wednesday's game and hopes to advance to a side session by Thursday. He hasn't ruled out pitching as early as Saturday in Chicago.

"It's time to push the envelope," said Wakefield. "We're running out of time."

After missing six weeks with a lower back strain that later turned into a nerve issue which caused weakness in his left calf, Wakefield returned to the mound on Aug. 26 and turned in seven dazzling innings (one earned run) in a win over the White Sox.

But two days later, his back had again locked up to the point that he had to be scratched from Tuesday night's start at Tampa Bay.

Wakefield would like to find out as soon as possible if the cortisone shot made a significant difference.

Because, if not, he told the Providence Journal that he could immediately undergo surgery to have the root of the recurring pain -- a loose fragment -- removed and still be back in time to pitch in the postseason.

"Medically speaking, there's more fluid in there now and it's pressing against the nerve, which is causing weakness and numbness," said Wakefield.

Red Sox manager Terry Francona admitted it has been a difficult injury to gauge. How soon can Wakefield pitch again?

"I don't know," Francona said. "Maybe not that far off. This is a hard one for us. He could potentially probably be pitching pretty soon and he could potentially probably not. You guys are seeing the game stuff we are. We're trying to make the right decision. It's hard because we know he can go out and get outs. We'll just continue to keep an eye on him."

While the Red Sox have two stalwarts at the top of the rotation in Josh Beckett and Jon Lester and Clay Buchholz emerging, they've lacked depth in the lower portion of the rotation. Daisuke Matsuzaka has been on the disabled list since June 20. John Smoltz and Brad Penny were both recently released following weeks of subpar performance. Paul Byrd recently came out of retirement and gave the Red Sox a quality outing to beat the Blue Jays. Junichi Tazawa, a 23-year-old callup, has been hit or miss.

The Red Sox would love to get Wakefield back on track. The right-hander pitched so well in the first half that he made it to the All-Star team.