fun·gi·ble - ˈfənjəbəl - adjective
(of goods contracted for without an individual specimen being specified) able to replace or be replaced by another identical item; mutually interchangeable.
late 17th century: from medieval Latin fungibilis, from fungi ‘perform, enjoy,’ with the same sense as fungi vice ‘serve in place of.’
1) Because reliever performance fluctuates wildly from season to season, most relievers are fungible;
2) Because this is an era of pitcher dominance, the difference between an above average reliever and an elite reliever is smaller than it was 10-15 years ago, so modern relievers are more fungible than any point in the last 15 years;
3) Because this is an era of enhanced defense, both in terms of opportunity for elite defenders with weaker bats and for defensive shifts (non-standard defender deployment), strategy makes the relievers more fungible than they have been in recent history;
4) Because team control lasts for 6 seasons, and because most relievers are failed starters, team control almost always lasts through the prime years (27-30), and because the game is more flush with cash with which to reward free agents than at any point in history, relievers who reach free agency are more fungible than their younger counterparts.
By WAR, David Robertson is the 4th most valuable reliever in all of MLB since 2009. But he's 8th since 2012, 9th since 2013, and he was 14th in 2014. Anyone who has watched Houdini work knows that he has not regressed. He is definitely a pitcher at the peak of his power. But the rest of the league has passed him by.
Maybe we'll look back at this as the true golden age of the pitcher, maybe there are multiple relievers today who will be elected to the Hall of Fame, and maybe we'll look at the 60s or the Deadball 1910's as distant eras and precursors.
Look: maybe at some point, we'll be able to say with certainty, this was the golden era of the reliever, with Robertson and Kimbrel and Papelbon and Chapman. The way the 50s was the golden era of center fielders and the 90s was the golden era of shortstops, maybe we'll look back at this as a unique moment, when a collection of truly elite pitchers changed the way we think about pitching.
But more likely, we'll look back at this era as an era of transformation because teams finally gave up their loyalty to statistics and past success, and teams began giving their loyalty to predication and future success.
If you've been following the developments around baseball in the last ten years, you probably agree with me that pitching hasn't gotten better individually, but teams have gotten better at identifying what works to separate the okay pitchers from the very good ones. And if you agree with me, then you agree that David Robertson is not irreplaceable.
I agree with the Yankees' strategy the last several years: if you're going into the free agent pool, dive into the deep end. Don't just sign Sabathia and Burnett; sign Teixeira too, because in for a penny, then in for a pound. And if it's a weak class, and the 2014-2015 offseason is an especially weak class at all the positions the Yankees need, so much better to let your entirely fungible reliever leave for another team and collect all the sweet, sweet draft picks you can manage.
It's not about whether David Robertson is a great reliever. It's about whether a great reliever can make a difference to this team. This team needs a shortstop, not a shut down reliever.