In an impossible instant, wild exuberance became implausible heartbreak Saturday, as what many in baseball had never seen but feared could someday happen, happened.

First baseman Kendry Morales, the Angels’ best slugger, had just swung the team to a 5-1 victory over Seattle with a 10th-inning, walk-off grand slam.

As he rounded the bases, a sun-splashed crowd cheering his every step, his teammates sped from the dugout to greet him at home plate, as walk-off custom dictates.

But that will be the last walk-off celebration to take place here for some time, for when Morales leaped into the cheering scrum of teammates to touch home plate, he landed awkwardly, immediately falling to the ground with, X-rays later showed, a fracture in his lower left leg that probably will end his season.

Morales will undergo surgery Sunday performed by Dr. Phillip Kwong, a foot and ankle specialist with the Kerlan-Jobe Clinic.

As Morales lay still, his teammates stood silent, many looking down in disbelief. Some quickly waved for trainers. The crowd’s cheers were silenced. The announcer’s call of final pitching statistics echoed through the quieted ballpark.

It was several minutes before a cart bearing a stretcher reached him. Morales, who was leading the team in batting (.290), home runs (11), total bases (94) and runs batted in (39), waved his arms toward the fans as he was driven away.

“It’ll change the way we celebrate,” Manager Mike Scioscia said. “It sure is exciting, but you always wonder if it’s an accident waiting to happen.”

Morales’ grand slam came with one out, after Maicer Izturis hit a double, Bobby Abreu was intentionally walked, and Reggie Willits reached on a grounder that was misplayed by former Angel Chone Figgins at second base.

Morales swung on the first pitch from Seattle reliever Brandon League, and it sailed to center field for what would have been a game-winner even if it hadn’t cleared the fence.

Scioscia said Morales didn’t land on anyone’s foot and that it didn’t seem to be anyone’s fault. “It’s a lesson for all of us,” he said. “It’s unfortunate.”

But for a team already struggling with a 24-27 record, there seemed no more unfortunate a blow than losing Morales the way they did.

Indeed, the Angels’ playoff chances may have left the field with the 26-year-old because no other player in their system compares with his talent and no other in their farm system seems talented enough to trade for a middle-of-the-order bat.

Angels General Manager Tony Reagins said players probably never will rush home plate to celebrate a walk-off win again: “If you go through this experience, it’ll speak for itself.”

Jered Weaver, who threw seven solid innings before earning a no-decision, agreed. “You’ve got to put it in the back of your mind now,” Weaver said. “You’ve never really seen anything like that happen.”

The closest incident, perhaps, came in 2002 during the American League division series, when the Minnesota Twins stormed the field in celebration and second baseman Denny Hocking, who caught the game’s final out, was caught beneath a pile of teammates.

His hand was stepped on and injured, causing him to miss the entire AL Championship Series against the Angels.

Angels center fielder Torii Hunter, who was also injured Saturday when a 95-mph fastball hit his left hand, leaving a soft-tissue contusion, was on that team, and said the situations are nothing alike.

“That wasn’t as bad as this,” Hunter said. “This is bad. Denny was our utility player. Kendry Morales is an All-Star, damn near.”

baxter.holmes@latimes.com