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. Despite racking up 18 holds, which ended up being second on the team behind only Kyle Barraclough, Phelps’ ERA was over a whole run higher than in 2016, his WHIP was 0.3 higher, and he let six close leads slip away.
As a result of the lower production, and due to the combination of a strong 2016 and high demand for impact relievers at the Trade Deadline, Phelps was traded to Seattle for prospects Brayan Hernandez, Brandon Miller, Pablo Lopez, and Lukas Schiraldi. After just ten appearances for the Mariners, Phelps was shut down for the season with an elbow injury.
Seeing as though David Phelps was only with the team for half of the 2017 season, and he only had a limited impact while in Miami before being dealt, it seems appropriate to briefly discuss the progress of the prospects acquired from Seattle, as they will potentially end up having a bigger impact on the franchise down the line.
Hernandez, now the Marlins’ number 12 prospect, is still regarded as a possible five-tool outfielder, and he finished the season hitting .263 over 49 minor league games, mainly at the short season Class-A level. It will be a few years before Hernandez is even close to the major leagues, and more will be learned about his potential next season, when he figures to spend a lot of time at Single-A Greensboro.
Miller (25th ranked prospect) saw limited playing time once he was assigned to Greensboro after the trade, and posted an ugly 8.86 ERA over five starts. However, for the season he went 9-7 with a 4.56 ERA over 23 Single-A starts, but all of that may come to mean nothing as a tendency to throw flat pitches may lead to a bullpen role in the future. The hope is still there, though, that Miller could become a fourth or fifth major league starter.
Lopez (27th in Miami's farm system) enjoyed success after being assigned to Class-A Advanced Jupiter as he produced a 2.18 ERA over eight appearances (six starts) to go along with 32 strikeouts and only seven walks over 45.1 innings. Lopez has great command and uses his pitch repertoire effectively, which bodes well for future success. Lopez missed all of 2014 and most of 2015 because of Tommy John surgery, and he is still regaining velocity, but once he does he could become a back-end starter for the Marlins as soon as 2019.
Schiraldi (not ranked) posted average numbers as a Class-A Advanced reliever in 2017, and appears to have a long road ahead of him to get to the majors. Never say never, but it is unlikely that Schiraldi will be a ranked prospect for the Marlins once the system is updated.
In conclusion, Phelps was a decent reliever for the Marlins in 2017 before being traded in July, but his season in Miami will be best remembered for the prospect haul the team received for him, if they turn out to be major league contributors in the future, that is.