FORT MYERS, Fla. -- Though Brad Penny described his right shoulder as feeling "pretty good" on Tuesday morning, he will start his exhibition season later than planned.

The original plan was for Penny to start for the Red Sox on Thursday afternoon against Puerto Rico's World Baseball Classic team. But after Penny felt some fatigue in his shoulder on Sunday, cutting a batting-practice session to 25 pitches, the Red Sox re-evaluated his program.

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Clay Buchholz, already scheduled to piggyback Penny on Thursday, will now start that game.

For the time being, Penny will go back to flat ground and focus on the ongoing process of strengthening the shoulder that limited him to just 19 games for the Dodgers in 2008.

"I don't think the shoulder strength is quite where it needed to be yet for the workload that had been scheduled out," said Penny. "It's better to take it easy right now than to get out there the first week of April and have it flare up on me. So [we're] just staying on the shoulder program -- [I'm] still going to play catch and just try to get my shoulder strength to where I need it to be."

Red Sox manager Terry Francona viewed the modified program as more of a conservative adjustment than any type of setback.

"After his side [session] the other day, we kind of sat down -- [pitching coach] John [Farrell] and I and [assistant trainer] Mike Reinold -- and said, 'OK, are we approaching this the best way we can?' And the answer was no," Francona said.

"We talked to Brad and we said, 'OK, look, this is a long year, we think maybe we can make some more strides by keeping you on flat ground and in the bullpen for just the foreseeable future -- not a long time and getting a little bit more strength, almost like putting gas in the tank.' And I think he was open to that. So that's kind of where we are."

With Spring Training roughly a week longer this year because of the World Baseball Classic, Penny is comfortable with the adjustment in his schedule.

"This is a good year to have the extra time, I guess," Penny said. "I'm just going to keep working and doing the shoulder program, and I'm pretty sure I'll get there."

Penny stressed that the current fatigue in his shoulder is nothing like the pain that made his 2008 season pure misery.

"No, not really," said Penny. "Last year was pretty bad for me, so it's not like that."

Francona said that it's too early to tell if Penny will start the regular season on the active roster, nor did he think that was particularly crucial.

"What we're really trying to do is get him healthy so he can go out and pitch like he can pitch," Francona said. "So for me to sit here and go, 'Oh God, I'd like to get him for that third game,' would be a little bit of the wrong message to send. We want to give this guy the best chance possible to be as strong as he can."

The Red Sox have an abundance of depth in the starting rotation. If Penny isn't ready to go right away, Justin Masterson, Buchholz and Michael Bowden are all capable of filling in.

Boston also has three off-days sandwiched in between the first 16 games of the regular season, meaning it can go with four starters at times if Francona chooses.