I'm not sure the bullpen situation is as dire as advertised, and FA RPs, as we've discovered, aren't always a better answer than your own pitchers.
RH: Neris, Pivetta, Hammer, Brogdon, DLS
LH: Morgan, Alvarez, Suarez
???: SD, Arano
Irvin has LOOGY potential, Garcia was called up too early but is a year older (all of 23).
Gilbert and Romero have LH potential, So do Singer and Davis.
Hernandez, Dohy, Russ and Dyer are later season potential call-ups. Llovera could rise quickly if moved to the pen.
With Howard, Damon Jones and Seabold good bets for the Lehigh starting rotation, DLS's future will probably depend on any progress with his secondary pitches, if not, he'll get moved to the pen. Russo and Eastman are AAA inning eaters, but they could allow a hard thrower like Llovera to move to the pen.
Point is not that they couldn't use some veteran RPs, but unless you're talking a closer or setup man, it's not worth breaking the bank for a middle or long reliever type given the organization has been stockpiling bullpen arms. As we saw with Arano and SD before they broke down, young pitchers can give you a boost.