In a frightening scene at Tropicana Field, Desmond Jennings' second-inning liner caromed squarely off the left side of Happ's head in a loud "thwack!" 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 the ballpark 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 for about eight minutes.
Toronto manager John Gibbons stood on the mound as Happ was strapped to a backboard and immobilized. The left-hander 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 or Happ's condition were immediately available.
Happ's injury was the latest to a pitcher struck by a batted ball, and Major League Baseball has discussed ways to protect hurlers on the mound.
Oakland right-hander Brandon McCarthy was hit in the head by a line drive last September, causing a skull fracture, an epidural hemorrhage and a brain contusion that required surgery. He was released from the hospital six days later.
Not long after Happ was injured Tuesday night, McCarthy's wife, Amanda, tweeted: "Thoughts go out to Happ and his family. Such a scary moment."
Detroit pitcher Doug Fister was struck in the head by a batted ball 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 armour, 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, and the Blue Jays went on to a 6-4 victory. Several players around the majors tweeted their thoughts and prayers for Happ and his family.
"Take for granted how fast this sport can be, Especially for players that close. No reaction time!" Dodgers infielder Jerry Hairston Jr. wrote.
Happ began spring training without a spot in Toronto's projected rotation. He earned a starting role when the Blue Jays left struggling Ricky Romero behind in Florida to work on mechanics when the season began.
Happ was obtained by Toronto in a trade with Houston last July 20. He was placed on the disabled list Sept. 7 and missed the rest of the season due to a broken right foot.