Yesterday, the Chicago White Sox and the New York Yankees engineered a trade that solidified the Yankees as contenders.

In exchange for four prospects -- among them 2016 first-round pick Blake Rutherford -- the Yanks received relievers David Robertson and Tommy Kahnle as well as third baseman (and sometimes first baseman) Todd Frazier.

In his second tour of duty with the Yankees, Robertson, will be a key piece of this deal, what with his 7 years, 402 games, and 393.1 innings pitched with New York.

New Jersey native Frazier will be an nice marketing tool for the Yankees' front office -- and the fact his 103 wRC+ is a significant upgrade over what the Yankees have had at both third base (87 wRC+) and first base (66 wRC+) doesn't hurt. There is a chance he's a rental, though, as he enters free agency this winter.

Robertson and Frazier will get the ink as the headline acquisitions, but the best player the Yankees acquired in this deal is Kahnle. And it's not particularly close.

A Breakout Campaign

Kahnle's career has had some interesting twists and turns. The New York native was originally drafted by the Yankees, and rose through their system before being nabbed in the Rule 5 Draft by the Colorado Rockies prior to the 2014 season. He spent 2014 and 2015 with the Rockies, and he was subsequently dealt to the White Sox after the 2015 campaign.

After an okay first year in Chicago, he exploded this season, posting what would be career bests in ERA (2.50), FIP (1.46), and xFIP (1.62), not to mention an impressive 1.62 SIERA.

Kahnle's current ERA slightly edges out his 2016 career-best mark of 2.63 ERA, but his FIP and xFIP demolish his previous career bests of 4.46 in 2016, and 4.11 in 2015. His career-high strikeout rate was 25.2%, which he's in the process of destroying with a 42.6% strikeout rate. He also walked a ton of people early in his career, with walk rates as high as 16.8% (2016) and 18.1% (2015), but this year he's dropped that down to an impressive 5.0%.

Not only has he enjoyed his best personal season, he's been one of the best relievers in all of baseball, and, statistically speaking, would own the Yanks' bullpen.

When comparing his numbers to his teammates in New York's 'pen (with a minimum of 20 innings pitched, which allows us to include Aroldis Chapman), he comes in at the top in every category. His 42.6% strikeout rate barely edges out Dellin Betances' 42.5% clip, but the rest of the numbers lead his new team rather comfortably.

His 1.62 FIP outdoes Chapman' 1.82 FIP, and his 1.62 xFIP is significantly higher than Chapman's 2.87 xFIP. His walk rate edges out Jonathan Holder's by 2%, and his 1.4 fWAR is a bit higher than Adam Warren's 1.2 fWAR.

Swing and Miss

Kahnle has to have some serious swing-and-miss stuff in order to have such gaudy strikeout numbers. His swinging-strike rate is up to 16.6%, in contrast to his career 12.7% mark. One of the most important aspects of his breakout campaign is that he's attacking the zone early at a much higher rate. His first-pitch strike percentage is 63.8%, nearly 10% higher than it has ever been.

He's also causing more swings in general, which makes sense, considering he's getting ahead in the count more often. His percentage of swings on pitches outside of the strike zone is up to 38.3%, notable because it's never been higher than 28.9%. The percentage of swings he's getting on pitches in the strike zone is 71.3%, and his overall swing percentage is up to 54.6% -- it has never been higher than 47.3%.

Kahnle's contact made from pitches outside the strike zone is down 10% from last year, coming in at 57.4%, and he's allowing contact on pitches in the zone a mere 76% of the time, which would be a career low. This is a perfect storm for Kahnle -- he's throwing more strikes, causing more swings, and generating more misses.

There is one drawback to his season, however, and it explains his rather high BABIP (.352) -- Kahnle is getting hit harder than the average reliever and significantly harder than he ever has been hit. He is allowing a 38.2% hard-hit rate, which is a little less than 10% percent higher than his career average and approximately 7% higher than the league-average reliever. He has a 24.7% line-drive rate, roughly 5% percent higher than the league-average reliever.

Team Control

One of the more valuable things about Kahnle is that he s under team control for almost the next half-decade. He's reaching arbitration for the first time this offseason, and he will not be a free agent until 2021, when he'll be 31. He's due for a raise, but it will hardly break the bank, especially for the traditionally free-spending Yankees.

While Frazier and Robertson are good players and are certain to help the Yankees in the stretch run -- and in Robertson's case, next season, as well -- the true star of this trade is Tommy Kahnle. He's evolved into an elite reliever, and, at only 27, looks to be entering his prime, all of which is why when we look back on this deal, we might well view Kahnle as the Yankees' biggest get.