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07/05/12 9:21 PM ET

'He's changed everything': Rizzo boosts Cubs

ATLANTA -- Since he was promoted June 26, Anthony Rizzo has reached base in each of his first eight games, he's hit safely in seven of those games, and is 5-for-12 with two home runs. Does Cubs manager Dale Sveum ever think about what it would have been like if Rizzo was on the team at the start of the season?

"I'd be lying if I said I didn't," Sveum said Thursday. "It's obviously a more dynamic offensive team. We're getting to that plateau a little more of scoring four or more runs in a game, which gives you a heck of a lot better chance."

The Cubs are averaging 5.4 runs per win this season, and averaging 2.6 runs per loss. They're also 6-2 since Rizzo arrived.

"Sure you think about [what might have been]," Sveum said. "You think about what if [Luis] Valbuena was here all year, being able to produce like he has and the defense he has. You think, but that's not how baseball works."

Valbuena has taken over at third base for injured Ian Stewart, who will have surgery on his left wrist Tuesday and is likely done for the season. Valbuena's only batting .215, but he's driven in 13 runs in 19 games.

Rizzo's presence has stabilized the Cubs' lineup, he's provided some power and he's smoothly taken over first base.

"Everything -- he's changed everything," Chicago's Alfonso Soriano said. "Batting third, hitting homers, defense -- I hope the way he plays and the way we play, I hope we continue that way in the second half."

Cubs to determine Sunday starter on Friday

ATLANTA -- Ryan Dempster could start Sunday in the Cubs' series finale against the Mets. Manager Dale Sveum said he wants to see how Dempster handles a bullpen session on Friday in New York.

"He's good enough to keep thinking about it," Sveum said Thursday, when asked if Dempster was good to go.

Dempster has been on the disabled list since June 16 with a sore right lat, and he threw his second bullpen session on Wednesday. Sveum will make a decision prior to Friday's game.

The Cubs' other option on Sunday is Chris Volstad, who struggled in 4 1/3 innings in Tuesday's 10-3 loss to the Braves. Sveum wasn't happy with Volstad not following the game plan, and the two talked.

"Everything's fine," Sveum said. "It was a good talk. It was a little bit about everything. It's just a matter of talking to him about letting the catchers do their job, and he agreed. He agreed that sometimes he gets a little out of whack on the mound and this and that, and we'll leave it at that. He agreed he needs to trust them, and not that he doesn't trust them, but he gets on his own path sometimes."

Volstad (0-7, 7.94 ERA) has gone nearly a calendar year without a win. His last came July 10, 2011, against the Astros.

Pencil him in: Castro a lineup mainstay for Cubs

ATLANTA -- All-Star Starlin Castro is baseball's iron man this year, and Cubs manager Dale Sveum has no plans to give his shortstop a day off.

Castro has started all of the Cubs' 82 games, including Thursday's series finale against the Braves. His only other challenger was Atlanta's Dan Uggla, who had Saturday off after starting the first 75 games of the season.

Castro, though, is 3-for-21 (.143) in his last six games, and entered Thursday's contest batting .291, a season low. His glove is keeping him in the lineup.

"That defense is saving us a lot of runs right now," Sveum said.

Castro, 22, who will be making his second All-Star appearance on Tuesday in Kansas City, never asks for a day off.

"You start seeing it in people's faces more than their performance," Sveum said. "When you have a defense that, knock on wood, is playing as well as ours right now, you don't want to do anything because of offensive [struggles]. We're playing too good now to think about that."

Extra bases

• All-Star Bryan LaHair and David DeJesus were both in the lineup Thursday against Braves lefty Mike Minor. It was a rare start for both, especially LaHair, who was 3-for-38 (.079) against left-handed pitchers this season.

"We'll see how he handles it," Cubs manager Dale Sveum said of LaHair. "He had some decent at-bats the last start [against a lefty]. No results, but the at-bats were better, the aggressiveness.

"It's just a matter of picking the right lefties to face," Sveum said. "At the same time, I still have to get [Jeff] Baker at-bats. It just might not be as consistent as it was because [Anthony] Rizzo is going to be playing every single day at first base. There won't be a whole lot of playing time at first base for the Bakers and [Joe] Mathers."

Rizzo was promoted from Triple-A Iowa on June 26, which bumped LaHair from first base to right field.

• High school pitcher Duane Underwood, the Cubs' second-round pick in the 2012 First-Year Player Draft, apparently will sign with the team. On Thursday, he tweeted, "Next week I will officially be a Major League Baseball player for the Chicago Cubs Organization #gocubs."

• A couple of days ago, Alfonso Soriano asked new hitting coach James Rowson how old he was. Turns out, the two were born in the same year, 1976, but Soriano's birthday is January, and he's 36. Rowson won't turn 36 until September.

"It's the first time I have a coach who's younger than me," Soriano said. "Now I know I'm a veteran in this game."

Carrie Muskat is a reporter for MLB.com. She writes a blog, Muskat Ramblings, and you can follow her on Twitter@CarrieMuskat. This story was not subject to the approval of Major League Baseball or its clubs.