These aren't necessarily the players who just bring back the highest return. They are, however, the ones who should bring back the best value while not hurting the organization's depth or long-term positional plans too much.
1. SS, Jorge Mateo: There isn't a more dynamic electrifying player in the entire Yankee farm system than this Dominican native. With plus-plus speed, above average or better defensive potential, and the chance to not only become a solid hitter but one with average big league power too, it's easy for Yankee fans to dream about a Jose Reyes-like player at the shortstop position for the next decade. However, with 25-year old Didi Gregorius firmly entrenching himself as a comparable two-way player already at the big league level already, one who is still very much cost-controlled, there isn't a glaring need at the shortstop position now or even in the foreseeable future.
Not only does Gregorius' presence make Mateo pretty expendable in the short-term but the Yankees also have a glut of quality shortstop prospects at the minor league level as well that would help offset the loss of dealing Mateo in potential trade. The trio of Wilkmerman Garcia, Hoy Jun Park, and even Diego Castillo at the lower levels offer pretty significant ceilings of their own and at least one of them could be in a Mateo-like position prospect-wise a year from now, and the combination of Tyler Wade, Abiatal Avelino, and Thairo Estrada all offer the safeness of an eventual big league projection too.
In fact, not only would Mateo and his top-shelf talent bring back the most value and not only would trading him not hurt the depth of shortstops in the organization too much either but dealing him now could also help alleviate some of the impending playing time problems that are sure to plague the position currently. It would hurt to lose a player with his talent but dealing him now also makes the most sense in nearly every aspect when considering potential trade chips.
2. 1B, Greg Bird: Just as would be the case in potentially moving Mateo in a trade, dealing this former fifth round pick would absolutely sting overall, especially in light of his .261-11-31 showing in 46 big league games as a rookie this season. However, taking emotion out of the equation, dealing this sweet-swinging lefty now could make a lot of sense when considering all factors.
Barring an injury to incumbent first baseman Mark Teixeira in 2016, given the current roster construction with both Alex Rodriguez and Carlos Beltran not only signed for at least one more season each but needing some time at designated hitter too, as good as Bird has proven to be thus far and as better as he appears to be getting the fact remains that there is no concrete plan to get him into the everyday big league lineup. As it currently stands he seems penciled in for significant minor league at-bats next season and that might not be optimal use of one of the organization's best assets in the short-term.
Unlike dealing Mateo, however, the Yankees don't exactly have a ton of quality first base prospects that would help offset a potential Bird trade but it's not as if they don't have any depth at the position whatsoever either. The Yankees could help negate the loss of moving Bird by shifting former first round pick Eric Jagielo over to first base. A somewhat suspect defensive third baseman, Jagielo swings a very similar potent bat to Bird so the dropoff offensively would be very minimal and there would be some time to let Jagielo adapt to a new position too.
Trading Bird now is not an ideal scenario for the Yankees but he surely would bring back a healthy return, it would not hurt the short-term plans for the Yankees in 2016, and it's not like they don't have a viable backup option to replace Bird should the Yankees decide to deal him now.
3. 2B, Rob Refsnyder: There would be a pretty dramatic difference in return between dealing the two aforementioned prospects and this former fifth round pick back in 2012 but that doesn't mean the 2012 College World Series MVP has little trade value. While he just completed his worst statistical professional season this year, hitting just .271 with nine home runs and twelve stolen bases, ironically his trade value might not ever have been higher, especially considering he put up those solid numbers at the Triple-A level before hitting a more than respectable .302 with an .859 OPS in 16 big league games, and accumulated at-bats in some of the Yankees' most important games down the stretch.
Much has been publicly made of his defensive shortcomings at second base, almost to a satirical and exaggerated point, but the fact remains that most big league clubs are looking to upgrade their middle infield offensive production and would welcome a consistent bat like Refsnyder's with open arms, especially at his price tag. Just like with Bird though it's not like the Yankees have a stockpile of big league ready bats at the second base position and in a perfect world they might be better suited using Refsnyder as a big league option now.
The Yankees don't have a lot of viable internal big league ready second base options so they are really at a 'use him or lose him' point with Refsnyder. If the Yankees don't have the patience to allow him to learn on the job defensively at second base at the big league level even though he could be a plus offensive contributor at the position then it makes too much sense to deal him now and get something of value for him that could help the club in other areas. Sending him back to Triple-A again in 2016 not only seems like a waste of time but terrible use of its resources.
4. OF, Slade Heathcott: A year ago it would have been nearly impossible to deal away this former first round pick for anything of value considering his long injury history and the fact that he was removed from the 40-man roster before he was re-signed to a minor league free agent contract. A lot has changed in one calendar year, however. Sure he dealt with some more injury problems this year but he proved he be a very capable big league outfielder when he steps on to the field, hitting .267 in 64 Triple-A games before hitting .400 with just five strikeouts in 17 big league games.
As the John Ryan Murphy-Aaron Hick deal this offseason proves, most teams still find value in former first round picks with high ceilings who have yet to break out at the big league level. Dealing Heathcott now might not bring back a ton of value but it could bring back an important organizational piece either at the big league level or to help stockpile their minor league talent. He [like Refsnyder] is really at a 'use him or lose him' point too. With Brett Gardner and Jacoby Ellsbury already in the big league mix, outfielders with similar skillsets and traits, having Heathcott around is a bit superfluous from a roster construction standpoint; he might not have the most value but he's the one who arguably needs to be moved the most while there is still some value for the Yankees. The Yankees can't really afford to lose him for nothing.
5. OF, Mason Williams: The comparisons between Heathcott and this former 2010 fourth round pick are eerily similar. A year ago the Yankees could not have netted a proverbial bag of balls for Williams, especially on the heels of hitting a combined .234 his previous two seasons and seeing his prospect status erode to an all-time low. He bounced back in a big way this past season, however, hitting a combined .318 between Double-A and Triple-A before hitting .286 with four extra-base hits in a brief eight-game big league call-up and he has since resurrected his once top prospect status.
Like Heathcott too, trading Williams now might not bring back the highest return as his inconsistent production history does offer some cloud of doubt but he too offers a bit too much redundancy on the current big league roster given the presence of Brett Gardner and Jacoby Ellsbury, not to mention Slade Heathcott as well. He too fits into the 'use him or lose him' category; if the Yankees don't plan on having him as a part of their short-term outfield plans -- and that seems highly unlikely at this point given the fact that all four outfield spots seem locked in for 2016 -- trading him now and getting something of value for him makes the most sense.