GLENDALE, Ariz. -- In the matter of approximately 16 hours, Jake Peavy went from a veritable White Sox roster lock to almost certainly not being part of their Opening Day active ranks on April 1 in Cleveland.

"Ah, he will be in Cleveland on Opening Day, but I don't know if he's going to be on the roster," said manager Ozzie Guillen of his veteran right-hander, speaking after Sunday's 9-7 loss to the Dodgers. "I know for a fact he wants to be on the team, but we have to make sure when he's on the team, he's on the team for good."

Peavy was shut down on Sunday after talking with doctors about discomfort felt following an 83-pitch effort Saturday against Oakland in Phoenix, an ailment which appears to be rotator cuff tendinitis. The problem is not related to Peavy's ongoing and seemingly remarkable comeback from experimental surgery on July 14 to reattach the tendon that anchors the latissimus dorsi muscle to the rear of the shoulder, and he has been started on anti-inflammatory medication.

Setbacks were expected along the healthy road back to the Majors for Peavy, whose offseason drive helped put him ahead of schedule for this return. Peavy didn't want to put much stock in the setback theory, but with the discomfort building up in the right posterior shoulder from one Cactus League start to the next, the White Sox and Peavy decided to move in trying to cut short this problem.

"It's kind of been coming since that first time," said Peavy, mentioning he felt a little discomfort after his debut against the Angels on March 4. "It has been getting a little more uncomfortable as I keep going.

"We want to not let it mount into some bigger issue than it is. But at the same time, I'm thinking it's just part of getting back into the routine. I threw for an awfully long time without any setbacks.

"Once you start going close to game speed like I have, things can pop up," Peavy said. "You're asking your body to do exactly what you're going to do in the regular season. It obviously has caused a little bit of discomfort. It's a small step back, and we'll re-evaluate the situation in the next couple of days to see where we go from there."

Re-evaluation is one thing, but Guillen made it clear postgame how Peavy will not be on the mound for his scheduled start Thursday in Mesa, Ariz., against the Cubs. Phil Humber gets the nod, as the White Sox move to Plan B for the 2011 season's outset.

Some doubt existed as to whether Peavy would make his set start on Saturday in Phoenix, as Peavy missed important work days in between due to a severe bout with the flu still clearly plaguing him. Guillen voted against Peavy staying on turn, meaning he would also stay on turn to break camp with the team and start for the White Sox on April 6 in Kansas City.

That vote was overruled, or in more accurate terms, the intense competitor that is Peavy convinced Guillen to let him face the A's. That method of convincing also worked last year, when Peavy skipped one start in Pittsburgh and then faced the Nationals after battling shoulder inflammation.

This same convincing won't work again, as far as Guillen is concerned. It's not an expression of anger or disappointment aimed at Peavy, but more a protective measure for Guillen's player and his staff.

"Jake Peavy will pitch the day I tell him to pitch. He's not going to convince me," Guillen said. "I don't care, we went through it. When he tells me 'Skip, I'm ready to pitch,' I give him another couple of more days to recover. That's the way we do stuff.

"When you tell your kid don't do this and the kid is doing it, and you don't do anything about it, you are not doing your job, and that has happened twice. My job is to protect him and the organization, to make sure when he is out there.

"I respect him because he wants to be out there. I love when players want to be on the field. That's the best thing that can happen to any manager," Guillen said. "He's that type of guy who has the passion for the game. He wants to help. Sometimes when you want to help, you don't really help."

As much as Peavy wants to be with the White Sox from start to finish, he is in agreement with Guillen over not coming back until he's absolutely ready. That Plan B for Guillen has been in place since the start of Spring Training, so the setback on Sunday would not be considered a crisis situation.

If a trip to the disabled list is in order for Peavy, then he might miss just three or four starts before returning to healthy action.

"I can tell you this: When I get back to Chicago and on this team, I will be ready to go as much as I can be," Peavy said. "There will not be anything ailing me out on the field."

"Opening Day is too close the way he sounds, that's another thing," Guillen said. "The way he sounds we shouldn't be counting on him for Opening Day. We have the Plan B and we should stick with that plan. Don't rush and do something stupid when we don't have to do it."