(CBS) It’s no secret that the Cubs are in the market for a starting pitcher as the July 31 non-waiver trade deadline approaches. What’s more difficult to discern is exactly whom Chicago is willing to part with from its strong core of young players and prospects.

To that end, the Cubs have been dangling middle infielders Starlin Castro and Javier Baez as the centerpieces of trade talks, ESPN’s Jayson Stark reported. Their names have been included in preliminary discussions with the Phillies about left-hander Cole Hamels, but Philadelphia has expressed little interest in a deal surrounded around Castro or Baez.

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Both come with questions. Castro, 25, is currently in the midst of a poor season, batting .239 with a .582 OPS while serving as the big league team’s everyday shortstop. Many wonder if he can return to All-Star form, an honor he’s received three times, and his mental lapses remain worrisome.

Baez, 22, is nearing a return to the Triple-A Iowa Cubs after suffering a fractured left ring finger on June 7. He struggled in a two-month big league debut stint in 2014, hitting .169 with a .551 OPS and a strikeout rate of nearly 42 percent. He’s responded nicely at Triple-A this season, batting .314 with eight homers, 29 RBIs and a .922 OPS in 37 games. His strikeout rate sits at a far-less-atrocious 25.3 percent as well.

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In any potential deal, the Cubs will be wary of dealing a young player for a two-month rental. That’s why Hamels, 31, is appealing, as he’s under contract control through 2018 with an option for 2019.

Hamels is 5-7 with a 3.91 ERA and 1.25 WHIP this season.

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“I don’t think they’ll be in the rental market,” a baseball executive told ESPN about the Cubs. “So the value to them, with Hamels, is the three years (left on his contract). This isn’t just about selling out to go for the wild card. It’s about getting ahead of (the free-agent ace market) this winter.”