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8/30/2013 8:05 P.M. ET

Big series features MVP candidates Yadi, McCutchen

PITTSBURGH -- While there is a month before members of the Baseball Writers' Association of America will mail in their votes for the National League's 2013 Most Valuable Player, this weekend series in Pittsburgh features two of the top candidates.

Cardinals catcher Yadier Molina is not only a favorite to win his sixth consecutive Gold Glove Award, but also entered Friday as the NL batting title leader with his .333 average. The work he has done to help usher 12 pitchers through their rookie season has been noticed, too, even if it cannot be quantified with statistics.

Pirates center fielder Andrew McCutchen, though, is having a similarly spectacular season. He ranked third in the NL with a .323 batting average and third with an on-base percentage of .401. He and St. Louis' Matt Carpenter were tied for the NL lead with 51 multi-hit games entering Friday.

So who has the edge? It's hard to tell.

"My criteria is always the same, the baddest dude in the league," Pirates manager Clint Hurdle said Friday. "Should Molina get votes? Absolutely? Should our guy? Absolutely. You saw the different challenges when Molina isn't in the lineup, they're a different team. We'd be a different team without McCutchen."

"I would say that's a pretty good definition," Cardinals manager Mike Matheny said of Hurdle's descriptor. "I think it's the guy who brings the most value to his team, more than anything else. Maybe I'm saying that because I know what Yadi does for us. You have to take everything into consideration. There are a lot of players out there having some very strong seasons. And the player you're talking about in McCutchen is a phenomenal player in the outfield and does a lot of impressive things out there.

"But I would love for people to see -- and once again, this is very biased coming from a catcher -- the due credit given to a player who does what Yadi does behind the plate."

Last year, McCutchen and Molina finished third and fourth, respectively, in MVP voting. Both were Gold Glove winners. How September plays out for both individually -- as well as their teams -- could create separation in their candidacy.

Of course, Matheny will continue to stump for his catcher regardless.

"Yadi has had a terrific season," Matheny said. "When you start looking at value to a team, value to a league, the offensive numbers speak for themselves. He's having a great season. He's a clutch player. But what he does behind the plate, it's really hard for me to even explain the value he brings to our club and what he does for a team and what he does against the opposition. He's a force to be reckoned with behind the plate and he has developed into a top-tier hitter."

Kelly to start Sunday's series finale on regular rest

PITTSBURGH -- The Cardinals and Pirates have settled on starters for Sunday's series finale, with Joe Kelly getting the nod for St. Louis and Kris Johnson set to be called up from Pittsburgh's Triple-A affiliate.

With the benefit of Thursday's off-day, Kelly will be pitching on normal five days' rest. This rotation alignment also means that Adam Wainwright will start Monday's series opener in Cincinnati.

Johnson has made one Major League appearance, that coming in relief Aug. 18. The lefty allowed two earned runs in six innings of an extra-inning game before being sent down the following day because the Pirates needed a fresh arm.

The Cardinals are 15-20 in games started by an opposing left-hander.

Club yet to select which rookie will start Tuesday

PITTSBURGH -- A trio of rookie starting pitchers -- Michael Wacha, Tyler Lyons and Carlos Martinez -- are slated to rejoin the big league club Tuesday, the day after the Minor League regular season ends. Which of the three will start for the Cardinals that day is yet to be determined.

Manager Mike Matheny said that all three pitchers remain in consideration as the Cardinals figure out how to fill the fifth hole in their rotation. And all three are using short stints in the Minors to stay ready in case the call comes.

Martinez made a seven-inning start for Triple-A Memphis on Thursday, during which he held New Orleans to one earned run on five hits and a walk. Martinez struck out nine while throwing 65 of his 91 pitches for strikes. With Memphis in the hunt for a playoff berth, the Cardinals transferred Martinez to the Double-A roster after the game since the 21-year-old right-hander would not be able to pitch again before the end of the Minor League season.

To stay on a starting schedule, Martinez will throw a bullpen session with Springfield on Saturday. Wacha, who was optioned to Double-A after having to extend himself for 65 pitches in relief Wednesday, will throw on the side, as well.

As for Lyons, Matheny said the left-hander was expected to throw a few innings in relief for Memphis on Friday. That keeps him open as an option for Tuesday, too.

The Cardinals have two more off-days in their September schedule and can use those to skip the rotation's fifth spot again if they would like. Whether the Cardinals patchwork that spot or find a month-long fit will depend upon whether one of the three rookies can seize the opportunity.

"I think anybody would like for an opportunity to be given to a player and they jump in it and force your hand that that's the one you need to have," Matheny said. "But we'll see what we've got. We'll run somebody out there on the third [of September]. We have a great opportunity -- we're playing Cincinnati -- for someone to take advantage of it."

Worth noting

Jake Westbrook threw a bullpen session Friday afternoon, the second such throwing session he has had since being placed on the disabled list with a lower back strain. Westbrook said his back continues to feel better and said he's hopeful that he'll be ready to come off the DL on Sept. 6, his first date of eligibility.

• With a loss Thursday, Double-A Springfield was officially eliminated from postseason contention. The loss was the fifth straight for Springfield, which has a 64-70 record with four games remaining on the schedule.

• The Cardinals entered this weekend series against the Pirates having won each of their last five series. The club is 11-5 during that stretch and has made up four games in the division standings. The string of winning series is the longest since St. Louis took six in a row in May.

Jenifer Langosch is a reporter for MLB.com. Read her blog, By Gosh, It's Langosch, and follow her on Twitter @LangoschMLB. This story was not subject to the approval of Major League Baseball or its clubs.