2017 stats (with Miami): 47.0 IP, 2-4, 3.45 ERA (3.71 FIP), 1.34 WHIP, 9.8 K/9, 0.4 WAR (Baseball Reference)
After filling starting, long-relief, set-up, and closing roles for the Marlins in 2016, and excelling while doing so, David Phelps was used almost exclusively as a middle reliever/ set-up man in 2017. Admittedly, he looked less impressive, although it would have been hard to repeat the best year of his career to date.
By no means was Phelps a bad reliever (he was a lot better than both Brad Ziegler and Junichi Tazawa, who were getting paid a lot more), he just failed to live up to heightened expectations.