The Reds dropped injured ace Johnny Cueto from their division series roster Wednesday, replacing him with right-hander Mike Leake a few hours before Game 4 against the San Francisco Giants.

Major League Baseball granted permission for the move at 11:30 a.m., less than five hours before the first pitch.

Cueto pulled muscles in his side during the first inning of the series opener Saturday night. The Reds had been hoping their 19-game winner would improve enough to be able to pitch again, but the injury was still bothering him.

Cincinnati was reluctant to make the move with Cueto because it also leaves him ineligible to pitch in the NL championship series, should Cincinnati make it that far. The Reds lead the best-of-five series 2-1.

"It's tough taking your potential Cy Young guy out of your rotation," general manager Walt Jocketty said.

Jocketty said the Reds filed the paperwork for the roster switch shortly before 10 a.m. on Wednesday and got MLB's permission at 11:30. Leake, their No. 5 starter, had been told that the move was possible, so he should be ready to start.

The Giants went with left-hander Barry Zito, who has been at his best late in the season. San Francisco had won the last 11 games he started.

The Reds' medical staff determined that Cueto's injury could bother him into the start of the NLCS, which was a major factor in the decision to remove him from the roster.

"It was a very, very tough decision to make, but our medical staff felt at best, Johnny would be able to pitch maybe one game in the next series if we get there, if he were available for that series," Jocketty said.

The Reds' other option was to keep Cueto on the roster and start Mat Latos on only three days of rest, putting the pitching staff in flux. Latos will pitch Game 5 if needed.

"It's quite a blow but at the same time, you hope Leake steps up and pitches well," manager Dusty Baker said.

Cueto pulled up after throwing a pitch in the first inning of Cincinnati's 5-2 win in San Francisco on Saturday night. The injury was initially diagnosed as muscle spasms in the back, but a strain in his right side was later detected.

That's an especially troubling injury for Cueto, who turns his back to the batter before each delivery, putting a lot of stress on his side.

"It's kind of a quandary as far as the injury goes," pitching coach Bryan Price said. The Reds were concerned that if they sent him back to the mound before the injury was fully healed, he could cause worse damage.

"It's more of a muscle strain in the side that was very mild, but with his torque and the way he pitches and his twisting motion, we didn't want to take the risk of injuring him," Jocketty said.

The major setback comes at the end of a record-setting season for the Reds' rotation. None of the five starters got hurt, a franchise first.

Leake had to be pushed back a day during a series against Cleveland in June, when a stomach virus went through the team. Otherwise, the rotation had impeccable health.

"A lot of it, I think, is just good fortune and good luck," Price said.

Leake went 8-9 with a 4.58 ERA, making him the obvious choice to be left off the playoff roster. When Cueto got hurt Saturday, the Reds told Leake to start preparing to replace him if needed. He left the ballpark on Tuesday night knowing he might start Game 4.

"He knew there was a good chance," Baker said. "He's known for possibly a couple of days, not that he would be pitching but that he was preparing to pitch. He was studying the charts. He was looking at the game. He was in the video room."

Leake faced the Giants once this season, throwing the first complete game of his career for a 5-1 victory at AT&T Park on June 19. He also homered off Matt Cain in that game.