The Yankees bullpen will be considered one of the best in the game in 2018 even if they come back with the status quo from the 2017 season. However, we shouldn't expect the Yankees to stick to their guns.
The Yankees have a strong collection of right-handed set-up relievers -- Dellin Betances, Chad Green (for this exercise let's assume he sticks in the bullpen instead of landing in the rotation), Tommy Kahnle, David Robertson, and Adam Warren -- and a left-handed closer (Aroldis Chapman) slotted into the bullpen for 2018.
Assuming the Yankees will use an eight-man bullpen as often as possible next season, they'll add a long man and likely a left-handed reliever to the mix. They will have a slew of options for the long-man role, and Chasen Shreve is the incumbent for the left-handed middle relief spot.
Shreve tossed 45 1/3 innings (46 appearances) in 2017, to the tune of a 3.77 ERA (120 ERA+), 1.32 WHIP and 58 strikeouts. Shreve, 27, is basically a garbage time reliever that has huge home run issue (1.6 HR/9 in 2017). As such, the Yankees might look to upgrade in one of few spots that could use it on the roster.
A trio of left-handed relievers that the Yankees could discuss as options sit on the free agent market (MLB Trade Rumors contract estimate in parenthesis) and one enticing lefty may be available via trade...
Jake McGee (three years, $18 million)
McGee was once a dominant reliever regardless of his delivery hand, but battled some injuries as he completed his time in Tampa. After a lackluster inaugural campaign with the Rockies, McGee bounced back to form in 2017.
He threw 57 1/3 innings last season, generating a 3.61 ERA (139 ERA+), 1.10 WHIP and 58 strikeouts. McGee, 31, has a proclivity to allow fly balls, which may not make him the greatest fit in Yankee Stadium despite being a lefty. Also, it is difficult to see the Yankees making a three-year commitment to McGee as he approaches his mid-30s.
Mike Minor (four years, $28 million)
Minor was once presumed to be on his way to a solid career as a starting pitcher with the Atlanta Braves, but suffered a shoulder injury, which ultimately required surgery. The surgery ended up taking two MLB seasons away from Minor (he threw just 42 1/3 innings in the Royals' minor league system in 2016). The Royals used Minor as a reliever in 2017 and he shined.
Minor hurled 77 2/3 innings last season, with a 2.55 ERA (176 ERA+), 1.02 WHIP and 88 strikeouts. Minor, who turns 30 in December, is also not a ground ball pitcher (37.2 percent in career). Further, Minor has had difficulty with home runs in the past, though he fared well in that department in 2017 (0.6 HR/9). Minor would present an interesting option for the Yankees in that he could supply multi-inning efforts, which could be a balance to Green's ability to do the same as a righty.
Tony Watson (two years, $12 million)
Watson, like McGee, may have his best days behind him, but that does not mean he cannot fill a need with an MLB squad. He made a name for himself with the Pirates, putting together some elite seasons, but has seen his effectiveness diminish the last two seasons.
In 2017, the Pirates finally traded Watson, with the 32-year-old landing with the N.L. champion Dodgers. Watson threw just 20 innings for the Dodgers (24 appearances), but he had more success (2.70 ERA, 1.05 WHIP, 8.1 K/9) than he experienced with Pittsburgh (46 2/3 IP, 3.66 ERA, 1.52 WHIP, 6.8 K/9). Watson allowed 1.3 HR/9 in 2016 and 1.2 HR/9 in 2017, which is a big strike against him.
On the trade front, Padres closer Brad Hand is a potential target for the Yankees. Hand was surprisingly kept at the trade deadline last season and he rewarded the Padres with an exceptional second half locking down ball games.
Since converting to a full-time reliever with San Diego, Hand has been among the best in the game, let alone as a southpaw. In 2017, he hurled 79 1/3 innings (72 appearances), posting an impressive 2.16 ERA (193 ERA+), 0.92 WHIP, 11.8 K/9, along with 21 saves (26 chances). The Yankees would have to part ways with upper-tier prospects to land Hand, but they might be more inclined to do that for a player at the height of his game and entering his age-28 season with two years of control left before free agency.
The Yankees are by no means in dire need of a lefty reliever because their righties can handle batters from both sides of the plate. However, it seems that having a southpaw reliever like Minor or Hand (those that can handle lengthier outings) would provide an upgrade. Having a rock-solid lefty would pay dividends should the Yankees reach the postseason again in 2018, where either Minor or Hand would surely be more beneficial than Shreve.
The Yankees built a farm system that has players ready to help internally and with the notion they could utilize depth to aid the major league squad via the trade market. In my view, they would be wise to inquire about Hand first and if it doesn't pan out check in on Minor -- though a three-year deal would be much easier to swallow.