As the going rate for elite relievers continues to make grown men and women blush, it’s increasingly evident that there’s value in not needing to get into the market for the crème de la crème of relievers. Through the hefty prices paid in the acquisitions of Craig Kimbrel, Ken Giles, Aroldis Chapman and, now, Andrew Miller, contending teams are making it clear that they value having that lights out guy at the back of the bullpen perhaps even more than we may have once thought. Fortunately for the St. Louis Cardinals, when they lost their closer, Trevor Rosenthal, first to under-performance and then (perhaps not coincidentally) to a rotator cuff injury, they had an internal alternative which kept them from needing to wade into the deep end of the relief pitching market.

Seung Hwan Oh has been absolutely dominant for the Cardinals this season first as a set-up man and, for the last month, as a closer. The 34-year-old right-hander who had been tremendously successful in both South Korea and Japan has posted a 1.69 ERA and a 26.4 K-BB% since being signed by St. Louis this past winter. His 1.94 FIP ranks tenth among relievers in baseball this season. With the loss of Rosenthal, the Cardinals could have pursued the top names on the relief market this month, but Oh gave them the freedom not to. Instead, they made a relatively quiet transaction this morning picking up left-handed reliever Zach Duke from the Chicago White Sox in exchange for 23-year-old outfield prospect Charlie Tilson — a deal which was announced in true old-school fashion by the teams themselves.

In 2014, Zach Duke posted a surprisingly strong season out of the bullpen for the Brewers. He was 31 at the time and it was his first full season in a major league bullpen after scuffling along as an under-performing starter for most of his 20s. That one great season led to the White Sox giving him a 3-yr/$15M which was heavily mocked at the time due to his age and lack of a successful track record. It would appear, however, that the White Sox were either on to something or extraordinarily lucky as Duke has continued to be a solid reliever since signing the contract.

Since the start of the 2014 season, he has thrown 157 innings to very impressive results: 2.87 ERA, 27.9 K%, 58.2 GB%. He walks more batters than you might like — 10.0% walk-rate — but it’s hard to complain when the overall results are as strong as his. The key to his success has been a curveball which misses bats — the whiff-rate on his curve this season is 43.9% which ranks 12th of 41 MLB relievers (min. 100 curves) — and a sinker which induces grounders on 67% of balls in play. This has resulted in him posting an impressive combination of strikeouts and grounders over the past three seasons.

The one other key thing about Duke that’s worth noting is that he’s one of the few guys who alternates two distinct arm slots. He adds deception to his delivery by moving back and forth between a typical three-quarters slot and a true sidearm motion. As a fastball heavy guy whose fastball rarely cracks 90 mph, this deception can help him keep hitters off balance.

Duke joins a Cardinals bullpen that’s been quietly very effective — sixth in the majors in bullpen ERA and seventh in FIP — despite Rosenthal’s struggles. They already have a left-handed setup man in Kevin Siegrist who currently sports a sub-3.00 ERA for the second consecutive season. Although Duke doesn’t have particularly stark platoon splits, 15 of his appearances this season have lasted just one batter, which is the 3rd highest total in the majors. He can easily slide into a lefty specialist role for the team, but one who is more than capable of facing right-handers when necessary.

On the White Sox’s end of this, they clear a bit of 2017 payroll by getting Duke’s $5.5M off the books and add a speedy 23-year-old center fielder who is nearly major league ready. Charlie Tilson has put up impressive offensive stats at Double- and Triple-A over the past two seasons. This year he’s posted a 100 wRC+ with a 12.9 K% and 8.4 BB% while playing for Memphis. He hits for hardly any power, but if he’s able to get on base enough to utilize his speed, it’s easy to seem him with a major league future as a fourth outfielder, if not more. Chris Mitchell’s updated KATOH prospect rankings have him at #81 in the major leagues which isn’t all that surprising given his strong stat lines in the upper minors. Tilson was added to the 40-man roster this past winter and could be called up sooner than later in Chicago given that center field is in flux for them at the moment due to Austin Jackson’s knee injury.

As long as Chris Sale and Jose Quintana are still on the roster, it will be hard for the White Sox to go into full rebuild mode. Dealing a productive but non-elite reliever like Duke, however, is the kind of selling a team like the White Sox should absolutely be doing no matter whether they intend to compete in 2017 or not. Duke wouldn’t be a difference-maker for next year’s team and, while Charlie Tilson isn’t a difference-maker either, exchanging Duke for a position player with a full slate of team controlled years remaining is a logical swap.

As for the Cardinals, they successfully solidified their bullpen without paying the exorbitant costs associated with elite relievers right now. If Duke continues to be as productive as he’s been over the past few years, they’ve added a reliable bullpen arm through next season, but with just $5.5M owed to him next year, they haven’t locked themselves into a prohibitively costly situation. It’s hard not to like this deal for both teams involved.