Thomas was diagnosed with a bruised left wrist after exiting Monday's 5-2 win over the Diamondbacks, Martin Gallegos of MLB.com reports.
Thomas popped out to shortstop in his first at-bat of the night and then was plunked on the wrist in the third inning. He was able to play defense in the bottom of the third, but Lawrence Butler ended up subbing in for Thomas prior to the bottom of the fourth. The Athletics will likely evaluate Thomas further Tuesday morning before deciding if he'll be available for the second game of the series later that evening.