By Bruce Levine–

CHICAGO (CBS) — For most of Sunday evening, it appeared the Cubs would complete a series victory over the Cardinals on the strength of quality pitching.

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Right-hander John Lackey gutted out 6 2/3 innings before leaving the game with shoulder stiffness and a 3-1 lead. Then it all unraveled in the eighth inning.

Returning after being sidelined since Aug. 2 with a right triceps issue, setup man Hector Rondon gave up four hits, including Stephen Piscotty’s go-ahead three-run homer. That was followed one out later by a Brandon Moss solo shot.

Rondon was removed after five batters, registering only one out in what eventually was a 6-4 win for the Cardinals.

“They are a really good team with good hitters,” Rondon said. “I came into the game to be aggressive, and they got me. Like I said before, I need to make better pitches. And I will next time, but they got me today.”

Trying to become the fifth Cubs starter to reach double-digit wins, Lackey gave up just one run and took a no-decision in his 106-pitch performance.

Lackey was treated by the team’s medical staff and said his shoulder felt better after the trainers took care of him.

“I am OK,” Lackey said. “The extra bullpen session was probably not the best thing for me. (He twice warmed up in extra-inning games recently.) I have been a little tight since then. It’s nothing crazy. I have been out there before feeling a whole lot worse. Yeah, I will be all right.”

With reliever Pedro Strop out for four to six weeks with a meniscus tear and Rondon missing most of the past two weeks, the Cubs bullpen has suddenly become a concern. Sunday marked the second day in a row the Cubs had a late-inning bullpen meltdown, as rookie C.J. Edwards struggled mightily in a loss Saturday.

Even with a comfortable 12-game lead in the NL Central, Chicago is looking for additional bullpen help, internally and externally.

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“Not having Strop, we just have to figure things out,” manager Joe Maddon said. “We have to restructure this somehow. Yes, they got us late twice.”

Anthony Rizzo had given the Cubs an early lead with an RBI single in the first inning. He also hit a monster home run in the eighth inning off of the right-field video board and had three RBIs in the game.

With the win, St. Louis holds a 7-6 lead in the season series between the teams. They will play six more times in the regular season.

“Right now their offensive side is pretty prodigious,” Maddon said. “I never have seen Moss swing this well. He is that good right now. They have a lot of offensive momentum going right now.”

The main concern after the loss will be how Lackey feels Monday and beyond.

“When you talk to him, I am sure he will say he will make his next start,” Maddon said. “I just don’t know yet. I just know his shoulder stiffened up, and they treated it. He felt better afterwards. That is what I know.”

After twice getting warmed up in extra innings in recent weeks, Lackey said he’s done taking one for the team and being ready for emergency pitching until postseason.

“I have done it twice in my last four starts,” Lackey said. “Hopefully not until October (again).”

The Cubs have lost back-to-back games for the first time since losing two straight to the White Sox on July 25 and 26.

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Bruce Levine covers the Cubs and White Sox for 670 The Score and CBSChicago.com. Follow him on Twitter @MLBBruceLevine.