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Updated top 250 fantasy baseball rankings

There's less than one-quarter of the 2015 season to go, the diminishing chunk of remaining schedule resulting in quicker fluctuations in the rankings.

No longer can fantasy owners -- especially those in head-to-head leagues with upcoming playoffs -- be so patient with struggling players or those dealing with nagging injuries. It's not a hard-and-fast rule, but in a standard mixed league, anyone ranked outside the top 100 is fair game as a cut, if there's a handy player available as his replacement.

Ah, but then there's that key: his replacement. The replacement is critical to deciding your add/drop, and it's why we so often ask for the corresponding move when judging it. So if you're in a weak spot at any one position, today, here are picks at each position who could provide a surprisingly good amount of value for the remainder of 2015.

Matz

Catcher: Welington Castillo, Arizona Diamondbacks. Yes, his 21.4 percent home run-per-fly ball rate is probably unsustainable -- his career mark is 10.4 percent, after all -- but Castillo has always shown a penchant for power. He had .217 isolated power during his career in Triple-A ball, and .169 during the second half of 2014, when it seemed like he was blossoming as a big-league hitter for the Chicago Cubs. But belief in Castillo going forward is founded upon two things: One, the hitting-friendly nature of Arizona's Chase Field, and two, playing time, as he has 16 starts in the Diamondbacks' past 24 games. Considering how ordinary the catcher pool after the top 2-4 this season, Castillo has a legitimate chance at top-five status at the position the rest of the way.

Matz

First baseman: Ben Paulsen, Colorado Rockies. First base is a position at which you'd hope to have a more productive player, but for those looking to fill it on the cheap or in a short-term pinch, consider the handy-when-the-matchups-are-right Paulsen. He's a .282/.335/.496 hitter against right-handers, and .289/.348/.521 hitter at Coors Field, this season.

Matz

Second baseman: Chris Coghlan, Chicago Cubs. Profiled last week, Coghlan now qualifies at second base, and he remains a heart-of-the-order hitter for the Cubs, sandwiched nicely between Kyle Schwarber, Anthony Rizzo and Kris Bryant against right-handers. A .266/.348/.483 hitter against righties this season, Coghlan has enough pop and speed to be worth your while.

Matz

Third baseman: Nick Castellanos, Detroit Tigers. It's perhaps too late to scoop up the red-hot Castellanos, a .261/.346/.538 hitter in 35 games since the All-Star break, but he's trending upward of late, including his peripheral numbers that bode well for his 2016 prospects. Consider that during that second-half span, he has .277 isolated power, an 11.8 percent walk rate and 40.2 percent fly-ball rate. In his career up to that point, he had .130 isolated power, a 5.9 percent walk rate and 37.1 percent fly-ball rate. Castellanos seems to be driving the ball with more authority, and might be ready for a 20-to-25 homer season in 2016.

Matz

Shortstop: Eugenio Suarez, Cincinnati Reds. You could nominate Odubel Herrera, profiled last week, or Francisco Lindor, but Suarez is the greater unknown of the three. His is a play for plate appearances and in a prime role; he's the Reds' usual No. 2 hitter these days, generally batting between Brandon Phillips, Joey Votto and Todd Frazier. A .298/.331/.496 hitter in 35 second-half games, Suarez is capable enough in all five primary Rotisserie categories to be worth the rest-of-the-year plug-in.

Matz

Outfielder: David Peralta, Arizona Diamondbacks. I profiled this entire position in last week's column, and would also throw Rusney Castillo, Coghlan and Stephen Piscotty into this space. But Peralta gets the nod because of how easy it is to mix and match him: He's a .311/.370/.547 hitter against right-handers this season, those rates only slightly improved from what were also good numbers during his 2014 rookie campaign.

Matz

Starting pitcher: Nathan Karns, Tampa Bay Rays. With word that Karns won't face an innings cap, his fantasy value enjoyed a bit of a boost this week. Though his numbers aren't top-shelf, he has made a specific adjustment this season that might result in an upcoming statistical bump, akin to Nathan Eovaldi's success with the splitter during the past month-plus: Karns has continued to hone a knuckle-curve, which has enjoyed a 6.7 Run Value per FanGraphs (only 23 pitchers have greater than five with either a curveball or knuckle-curve during the 2015 season). Thanks in part to that pitch, he has a 25.2 percent strikeout rate in seven starts since the All-Star break, making him a sneaky source of K's in the upcoming weeks.

Matz

Relief pitcher: Shawn Tolleson, Texas Rangers. He's the No. 3 reliever on our Player Rater's Last 30 split, thanks in part to a 12-for-13 save conversion performance, 2.25 ERA and 27.8 percent strikeout rate in 20 second-half appearances. What's more, Tolleson has been one of the bright spots for what has otherwise been a mediocre Rangers bullpen. That helps him more than it hurts, as it means less competition for the role even if his setup men are more likely to blow leads in front of him, but at least the Rangers are a competitive squad sure to generate many chances. Much of Tolleson's success can be attributed to his changeup, which he introduced in 2014 and has used more heavily this year, giving him three above-average pitches (four-seam fastball, slider).

New ESPN position eligibility

The following players added new position eligibility within the past week. As a reminder, position players need to appear 10 times at a new position to gain in-season eligibility, while pitchers need to make three starts to earn starting-pitching eligibility or five relief appearances to earn relief-pitching eligibility.

Aaron Brooks (SP), Michael Cuddyer (1B), Kendry Flores (RP), Brock Holt (SS), Kyle Lohse (RP), Kris Medlen (RP), Brad Miller (OF), Josh Rutledge (2B), Eric Sogard (SS), Kelby Tomlinson (2B), Luis Valbuena (1B)

The following players are within two games of earning new eligibility, with their total games played noted at the listed position: Joaquin Arias (SS, 8 games), Daniel Castro (SS, 8 games), Starlin Castro (2B, 9 games), Ivan De Jesus (SS, 8 games; OF, 9 games), Jake Elmore (3B, 9 games), Eduardo Escobar (2B, 8 games), Ryan Flaherty (1B, 8 games), Logan Forsythe (3B, 8 games), Corey Hart (1B, 8 games), Cesar Hernandez (3B, 9 games), Brock Holt (1B, 8 games), Brian McCann (1B, 8 games), Will Middlebrooks (SS, 8 games), Daniel Murphy (1B, 8 games), Daniel Nava (1B, 8 games), Rey Navarro (2B, 9 games), Kris Negron (OF, 9 games), Derek Norris (1B, 8 games), Sean Rodriguez (3B, 8 games), Miguel Rojas (2B, 8 games; 3B, 9 games), Brendan Ryan (3B, 8 games), Luis Sardinas (2B, 8 games), Justin Turner (1B, 9 games), Henry Urrutia (OF, 8 games).

Going-forward rankings: Week 20

Listed below are my updated, going-forward rankings. These are based upon an ESPN standard league of 10 teams and Rotisserie 5x5 scoring. Click here to see these rankings sorted by position.