SAN FRANCISCO — Chris Carpenter had to figure there might be a hitch at some point along the way in his remarkable return.

The St. Louis Cardinals’ longtime postseason ace came back from a complicated operation that removed a rib and two neck muscles just to get on the mound again this October for the reigning World Series champions.

His recent run of spectacular playoff pitching ended in one rough inning as St. Louis lost Game 2 of the NL championship series to the San Francisco Giants 7-1 on Monday night and headed back home to Busch Stadium with the best-of-seven matchup tied at 1.

Carpenter surrendered Angel Pagan’s leadoff homer in the first, then made quick work of the Giants with consecutive 11-pitch innings before a mistake-filled fourth did in the Cardinals.

And, strangely enough on this night, Carpenter might have helped the Cardinals more with his bat than his arm. He hit an RBI double in the second inning off Ryan Vogelsong for St. Louis’ lone run.

In the fourth, Carpenter was charged with a throwing error when he fielded Brandon Crawford’s tough chopper to his left and made a rushed throw to first that was out of reach for Allen Craig. That allowed the go-ahead run to come home, and San Francisco added another when left fielder Matt Holliday misplayed Marco Scutaro’s two-run single and a third run scored on the play.

Carpenter’s night was done after that, a 29-pitch inning. The bearded right-hander took a seat in the dugout, stone faced and seemingly unfazed. He allowed two earned runs, five in all, and six hits in four innings — and his postseason ERA in two outings this year is still an impressive 1.86.

He certainly will be eager for another chance, and soon: Carpenter is ultra-competitive, one of the Cardinals’ most decorated players and their winningest pitcher in the postseason with 10 victories.

Carpenter played a key role during last year’s title run, going 4-0, with wins in the division series clincher and again in the decisive Game 7 of the World Series against the Texas Rangers.

The 37-year-old Carpenter had surgery on July 19 to cure numbness on his right side. He had put off the procedure back in 2008, when the symptoms weren’t nearly as bad and he was just plain scared to do it.

Once the numbness became constant, began moving up into his face and became far more than an annoyance that caused tingling in his arm and fingers, he had had enough. He went for the operation, knowing full well it would be months before he pitched again.

These days, he has a scar beneath his collarbone serving as a constant reminder.

“The Cardinals have invested a lot into me, and I felt an obligation to go ahead and invest my body back into them a little bit and see what happens,” Carpenter said.

After hours of grueling rehab, Carpenter made his season debut Sept. 21 against the Cubs at Wrigley Field. He threw all of 17 innings in three starts — all over the last two weeks — and that’s hardly even a spring training tuneup for a grind-it-out kind of guy accustomed to working 200 innings, or at least something close.

After Carpenter went winless in the regular season, there he was throwing shutout ball in an 8-0 Game 3 win in the division series against Washington last Wednesday — 5 2-3 scoreless innings to be exact.

While he insists he’s still fighting and relearning his mechanics at times, he has given Mike Matheny more than the rookie manager ever could have expected as together they try to get back to the World Series.

“He’s gone through so many things that a lot of pitchers, a lot of players would just say, ‘OK, you know what, this has gotten the best of me, maybe it’s time to hang them up, I won my World Series and all that,”’ Matheny said. “But he has such a desire to pitch and get through this. He’s a competitor, like I’ve never seen.”

Now, Matheny gives the ball to Kyle Lohse in Game 3 on Wednesday.

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