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St. Louis Cardinals manager Mike Matheny has been trying to pick his spots in terms of when to give 36-year-old first baseman Lance Berkman days off because of his history of knee problems.

Now, he may not have a choice.

Berkman left Saturday night's game with an injured right knee after stretching for a throw from shortstop Rafael Furcal on Justin Sellers' groundout for the third out of the second inning. Matt Carpenter took over at first and committed a fielding error on a hard-hit grounder by former Cardinal Adam Kennedy in the fourth, allowing the game's first two runs to score in Jake Westbrook's 6-0 loss to Clayton Kershaw and the Los Angeles Dodgers.

"You go up against a lineup like that and against a pitcher like Kershaw and you know that any kind of mistake is probably going to cost you. And it did," Matheny said. "That was a play that can be made — and most often would be made — but it didn't happen and it ended up costing us a couple of runs. Pitchers are going to start thinking that they need to do more when the plays aren't made behind them."

Westbrook (4-3) was charged with four runs — three earned — and six hits in 6 1-3 innings. The 34-year-old right-hander, pitching at Dodger Stadium for the first time in his 12-year career, departed after giving up Kershaw's opposite-field double off the glove of left fielder Matt Holliday. It was Kershaw's first extra-base hit after hitting 28 singles over five seasons.

Tony Gwynn Jr. greeted Eduardo Sanchez with a single that drove in Kershaw. The Dodgers got two more runs that inning when Gwynn scored on Sanchez's wild pitch and Andre Ethier added an RBI single that increased his NL-best RBI total to 36.

Westbrook knew he would have his work cut out for himself facing the Dodgers' ace, but he didn't let that distract him.

"You try not to do that, because you always want to go out there and throw a decent ballgame and put up zeros no matter who you're facing," Westbrook said. "If you put too much added pressure on yourself, it's really counter-productive. So you just go out there and pitch your game and do the best you can."

Berkman had just come off the disabled list last Sunday after being sidelined for 21 games because of a strained left calf he aggravated while chasing a popup. Another trip to the DL is a distinct possibility, judging by the way he was limping around in the clubhouse.

"I wish I knew," said Berkman, who has been on the DL five times. "We're just going to wait and see what the MRI says, but it just doesn't feel right. It buckled on me twice, and I don't know why it did that. I was able to move around OK in here, and then it buckled again. So I have to be careful with it. It's not too painful, but it's just a weird feeling. I wish I could describe it. There's a little bit of instability that's mildly concerning."

The switch-hitting Berkman, a six-time All-Star and the NL comeback player of the year in 2011, has played in only 13 games this season and is batting .333 with a homer and four RBIs.

In Friday night's series opener, he homered as a pinch-hitter against Dodgers closer Kenley Jansen with two outs in the ninth to tie the score, before the Cardinals lost in the bottom half on a bases-loaded walk. The homer ended the longest season-opening home run drought of his career after 40 at-bats.

The 12-year veteran underwent knee surgeries while with the Houston Astros — two on each. So needless to say, the defending World Series champions are waiting with baited breath on the results of his upcoming MRI exam.

"I'd be surprised if it was anything that major," Berkman said. "My guess is that I probably got a little meniscus sting. I think it's well-documented that my knees aren't perfect. They've had a lot of mileage, so from time to time, they get a bit cranky. So it wasn't totally unexpected."

Kershaw posted his fourth career shutout with a six-hitter. The reigning NL Cy Young winner earned the seventh complete game of his career on 116 pitches, striking out four and not allowing a runner past second base against an offense that came in leading the NL in team batting average, runs, homers, on-base percentage and slugging percentage.

"From the very first pitch of the game, we knew right away that they were going to be aggressive and swing the bats," Dodgers catcher A.J. Ellis said. "That's the tone a lot of teams have set against Kershaw this year, and tonight was no different."

Kershaw did not walk a batter for the third time this season while lowering his ERA from 2.22 to 1.90. The 24-year-old left-hander, who is 14-1 with a 1.40 ERA in his last 19 starts at home.

Sellers triggered a four-run seventh inning with his first home run of the season for the NL West-leading Dodgers, who improved the best record in the majors to 27-13. They beat the Cardinals for the sixth straight time — their longest winning streak against St. Louis since an eight-game stretch from July 24, 1975 through May 12, 1976.

Notes: In eight starts this season, Westbrook has yielded just one run over the first three innings. He has made only two other regular-season appearances against the Dodgers, both in relief for Cleveland in 2003.