As has already been discussed, Nippert disappointed pitching out of the pen for the D-Backs last year, posting a 5.56 ERA.
But his peripherals were actually pretty solid...he had a 3.90 DIPS ERA. While his BABIP was a shade over .300, which generally reflects some bad luck, his biggest problem appears to be his performance with RISP...overall, Nippert allowed opposing hitters a .267/.327/.417 line, but with RISP, it was .300/.386/.620.
Generally, one would expect that to be a matter of bad luck, rather than an inherent inability to perform in clutch situations.
PECOTA, Bill James, Marcel, Miner, and CHONE all project him to have an ERA below 5.00 this year, with only ZiPS slotting Nippert above 5.00.
Take that for what it is worth.