In a frightening scene at Tropicana Field, Desmond Jennings' second-inning liner caromed squarely off the left side of Happ's head, and 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.
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.
Jennings stood with his hands on his head, and other players were visibly concerned as they watched Happ receive medical attention. The left-hander was strapped to a backboard and immobilized before he was lifted onto a stretcher and wheeled off the field through an opening behind home plate.
Just before he disappeared under the stands, Happ raised his right hand and waved. He received a standing ovation from the crowd, and the game resumed after an 11-minute delay.
No other details about the injury were immediately available.
Happ's injury is the latest involving pitchers getting hit by batted balls, and Major League Baseball has discussed ways to protect hurlers from injuries.
Oakland's Brandon McCarthy was hit on the head by a line drive last September, causing a skull fracture and brain contusion that required surgery. And Detroit's Doug Fister was struck on the head during the World Series. Fister was unhurt and stayed in the game.
Major league general managers discussed the issue during their meetings in November and MLB presented several ideas at baseball's winter meetings only weeks later.
MLB staff have said a cap liner with Kevlar, the high-impact material used by military, law enforcement and NFL players for body armor, is among the ideas under consideration.
The liners, weighing perhaps five ounces or less, would go under a pitcher's cap and help protect against line drives that often travel over 100 mph.
MLB could implement the safety change in the minor leagues, as it did a few seasons ago with batting helmets, but would require the approval of the players' union to make big leaguers wear them.
Brad Lincoln replaced Happ.