Gallagher was diagnosed with a strained left hamstring after exiting Sunday's game against the Yankees.
Gallagher went 1-for-1 with an RBI single before departing the game in the bottom of the fourth inning following his second plate appearance, when he appeared to tweak his hamstring while running out a sacrifice hit. Though the Royals are labeling Gallagher as day-to-day, there's a good chance that he lands on the 10-day injured list.