Happ went down clutching his head after being hit by a scalding shot off the bat of the Tampa Bay Rays' Desmond Jennings in the second inning of the contest at Tropicana Field.
Blood was visible on the pitcher's head as trainers, medical personnel, coaches and teammates gathered around the pitching mound.
The left-hander was placed on a stretcher and wheeled off after several minutes.
He was conscious and talking and gave a wave to the crowd as he was being transported off the diamond.
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