Relievers are fickle. It’s become a truism in baseball, due mostly to the recent ubiquity of analytics. The numbers show that it’s quite tough to predict what one relief pitcher will do from one season to the next. For example, in 2012, Fernando Rodney posted a 0.60 ERA for the Tampa Bay Rays. This past season, he finished with a 5.68 ERA before the Seattle Mariners sent him to the Chicago Cubs. Neal Cotts: 1.11 ERA in 2013; 4.32 in 2014. Huston Street: 1.37 in 2014; 3.18 in 2015.
That being said, a handful of relievers have proven themselves to be reliable year in and year out. This list includes Mariano Rivera, Trevor Hoffman, and Billy Wagner. More recently, additions to the list have included Jonathan Papelbon, Greg Holland (before his injury), Aroldis Chapman, and Craig Kimbrel. Soon, we may be able to add Ken Giles to that list.