The Yankees have a reputation for being able to churn out effective young relievers over and over again, but the 2015 Yankees weren't so successful in that regard. Of course, they still had one of the best bullpens in baseball, thanks to former in-house products like Dellin Betances and Adam Warren, but most of their shutdown bullpen came from free agency (Andrew Miller) or shrewd trades (Chasen Shreve, Justin Wilson). When it came to new players coming up from the farm system, the Yankees got burned.
To be fair, for most of the season, the Yankees got incredible results from Miller, Betances, Wilson, Shreve, and Warren, so they didn't exactly NEED any kind of help.