Nelson was placed on the 15-day injured list Monday, retroactive to Aug. 5, with lower back spasms.
Nelson has been outstanding out of Arizona's bullpen this season, posting a 1.57 ERA and 0.90 WHIP over 34.1 innings, but the club will be without one of its most reliable relievers for at least the next couple weeks. Edwin Uceta was recalled from Triple-A Reno to take Nelson's place on the roster.