The ball went all the way into the bullpen in foul territory halfway down the right-field line. Happ dropped face down at the front of the mound, holding his head with his glove and bare hand.
The Blue Jays said the pitcher was taken to Bayfront Medical Center, where he was alert and undergoing tests, manager John Gibbons said.
Nursing supervisor Natasha Keller told The Associated Press that Happ had been admitted to the hospital and was in stable condition.
“I think the last indication was that he was alert and feeling better and had gone for a CT scan. That’s the last I heard,” Toronto pitcher R.A. Dickey said.
Jennings ended up on third base with a two-run triple. Team trainers, paramedics and medical officials rushed to Happ’s aid as Tropicana Field fell into a hush.
“It’s devastating. … I could barely watch it,” Dickey said. “You just don’t know what to think, really. It paralyzes you a little bit. And when it sounds like two bats, when you hear the sound off the bat and it sounds like it hits another bat, it’s scary. It’s really, really scary. I just started praying in the spot. That’s all I knew to do.”