With Tuesday's non-waiver Trade Deadline quickly approaching, contending clubs continue to canvas the market for pieces that could tip a division race in their favor or increase the chances of a deep October run. But getting coveted pieces also requires that some be given up.
Few players are considered truly untouchable, as most anyone can be made available at the right cost. There are, of course, exceptions: those rare players who because of contract status, future ceiling or current impact are certain to stay put.
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So who in the National League Central fits that description? Let's take a look:
Brewers
Who is untouchable: Ryan Braun
Why: The Brewers' window to trade Braun has closed. They came close to a deal with the Dodgers in August 2016, but the Brewers couldn't get it done in time, and Braun has since struggled with a series of injuries that have limited his availability and production. Barring a resurgence over the final 2 1/2 guaranteed years of his contract, there could be some uncomfortable decisions ahead.
Who is not: Corbin Burnes
Why: The Orioles wanted Burnes, Milwaukee's 23-year-old top pitching prospect, in a package for Manny Machado, and the Brewers said no. That led to a sense Burnes was untouchable. But that may not be true if it's the right kind of trade. Burnes would almost certainly have to be included in a deal for a top, controllable player, especially a pitcher. Think someone like Jacob deGrom, if the Mets go in that direction.

Cardinals
Who is untouchable: Jack Flaherty
Why: The Cardinals are loaded with young starting pitching, but Flaherty might have the highest ceiling of the group now that Alex Reyes is rehabbing from another injury. St. Louis sees Flaherty as an arm to build around, especially as Adam Wainwright nears retirement and Michael Wacha inches closer to free agency. Flaherty is putting together a season that should have him in the conversation for the NL Rookie of the Year Award at season's end.
Who is not: Carlos Martinez
Why: With Martinez locked down under a team-friendly contract through 2023, the Cardinals see him as a long-term ace. But with a plethora of young pitching, he might also be their best trade chip. Dealing Martinez would require the Cards be overwhelmed by an offer, but for a club that lacks high-ceiling, impact position players, it could make sense to flip Martinez to address that deficiency.

Cubs
Who is untouchable: Anthony Rizzo
Why: The Cubs don't officially have a captain but if they did, it'd be Rizzo. His offensive numbers this season were a bit down in the first half, but he's picked up the pace since moving into the leadoff spot, even as an unconventional top-of-the-order hitter. He's an NL Gold Glove Award contender, an on-field leader and arguably the face of the franchise.
Who is not: Addison Russell
Why: The Cubs aren't looking to part with Russell, but they could reshuffle their infield to deal with his departure if it's necessary to address another need. If needed, Javier Baez would be an option to shift to short. Russell's power numbers are down this year and he leads NL shortstops in errors, but Russell could be an attractive piece to other clubs given that he is under team control for another three seasons.

Pirates
Who is untouchable: Jameson Taillon
Why: Given their spending limitations, the Pirates need to develop front-line starters -- and they seem to have one in Taillon. He's under team control for four more seasons and is the closest thing the Bucs have to an ace now that Gerrit Cole is pitching in Houston. There's upside with Taillon, too, as he's unlocking some of his potential this season by being more willing to throw his curveball and slider.
Who is not: Felipe Vazquez
Why: Pittsburgh is highly unlikely to deal anyone with more than two years of control remaining, nor is it actively shopping Vazquez. But he may be the most appealing piece the Pirates have, and there's always the possibility that a club is willing to overpay for him. The Bucs have a history of trading closers (Joel Hanrahan was dealt for Mark Melancon, who was dealt for Vazquez), and the Brad Hand deal served as a reminder to how a cost-controlled closer can net impact prospects.

Reds
Who is untouchable: Joey Votto
Why: For starters, the 10-year contract extension Votto signed in 2012 came with a full no-trade clause. He has repeatedly maintained his desire to remain in Cincinnati and see out the rest of the rebuild. And even though he is due $25 million per year through 2023, the Reds are getting great value out of the deal with Votto continuing to produce at an elite level.
Who is not: Shed Long
Why: Long is the Reds' No. 8 prospect, according to MLB Pipeline, and is currently with Double-A Pensacola. A lefty hitter, the 22-year-old has emerged as one of Cincinnati's top prospects in only a few seasons and has developed his power. But the Reds could soon have a second-base logjam, especially if Scooter Gennett is given a contract extension. The organization also has No. 1 prospect Nick Senzel, as well as Dilson Herrera and Alex Blandino, as potential future fits at second.