Kontos had his contract purchased from Triple-A Columbus on Tuesday.
Kontos will replace the injured Evan Marshall (elbow) on the active roster and in the bullpen. The veteran reliever was let go by the Pirates earlier in the year after diminished velocity resulted in a brutal 5.03 ERA and just nine strikeouts across 19.2 innings. He'll likely start out in a lower-leverage relief role, but if Kontos is able to regain a few ticks on his fastball and return to last year's form (3.39 ERA across 66.1 innings), he could work his way into a more prominent spot in the bullpen.