Munoz was removed from Sunday's game against the Nationals due to lower-back tightness, Daniel Kramer of MLB.com reports.
The Mariners closer entered for the ninth inning of a 7-1 game since he hadn't pitched in nearly a week, and he allowed both batters he faced to reach before departing with the injury. The initial diagnosis is encouraging since it's not an arm issue, and Munoz should be considered day-to-day heading into Tuesday's series opener versus Baltimore