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8/19/2013 7:25 P.M. ET

Rizzo misses series opener due to illness

CHICAGO -- Cubs first baseman Anthony Rizzo was held out of the lineup on Monday because of illness, but manager Dale Sveum said he's hopeful the day and a half off will be enough time for the slugger to recuperate.

"He was pretty sick last night and pretty weak," Sveum said. "Migraine headache and the whole thing. Hopefully it all passes today and he's ready to go tomorrow for a night game."

Rizzo has appeared in 122 of the Cubs' 123 games this season, all but one as a starter (the other as a pinch-hitter). Each game has been at first base, and he's hit primarily from the No. 3 spot.

He's hitting .230 with a career-high 18 homers and 65 RBIs.

Catcher Dioner Navarro was batting third on Monday, and Cody Ransom was at first base.

Hoyer: Castro among most talented 'when he's right'

CHICAGO -- Though shortstop Starlin Castro has often received the brunt of the negative attention when it comes to the Cubs' miscues, his manager and general manager both cautioned Tuesday against placing too much emphasis on one single player for the team's struggles.

The two-time All-Star shortstop had a brain cramp on a fifth-inning popup in the Saturday afternoon loss to the Cardinals that allowed a run to score. Castro caught a deep infield fly from Matt Carpenter in left, but seemed unaware of Jon Jay on third base, who tagged and scored. Manager Dale Sveum benched Castro after the play.

"Our situation is where every day is an evaluation here to see where we're going to be in the future and where that particular player or all 25 players are going to line up in the situation next year," Sveum said.

General manager Jed Hoyer said Castro is one of the team's best players and -- with a seven-year, $60 million contract extension last year -- a major building block for the future. Because of that, Hoyer said, Castro sometimes receives a bit of "undue focus" on his struggles.

"I don't look at it like people trying to isolate him," Hoyer said. "I do think he made a really bad play the other day, he's had a few of those, and he has to stop doing that. I don't think anyone is forgiving that. But I think the way we look at it, he's having a down year. There's no reason in the world to think he can't get back to the way he played as a 21-year-old."

Castro hit. 300 in his rookie season in 2010, then .307 the following year. He's batting just .244 this season with an on-base percentage 44 points below his career average.

"As a staff, we have to get to the bottom of why he hasn't had as good a year, how we can get him better, and isolating him and focusing on him as a negative, that doesn't help that," Hoyer said. "This is a guy that, when he's right, is one of the more talented young players in the game."

Joey Nowak is a reporter for MLB.com. Follow him on Twitter at @joeynowak. This story was not subject to the approval of Major League Baseball or its clubs.