Surgery is set for Sunday, with Dr. Phil Kwong performing the procedure. He is a foot and ankle specialist with the Kerlan-Jobe Clinic.
There is no timetable for Morales' return, according to the club. He will be placed on the 15-day disabled list with a corresponding move set for Sunday.
A native of Cuba, Morales leads the Angels in batting average (.290), home runs (11), total bases (94) and RBIs (39). He had started every game at first base following a breakout 2009 season in which he batted .306 with 34 homers and 108 RBIs.
The Angels' players couldn't wait to celebrate Morales' blow, and they watched in horror as their slugging first baseman landed awkwardly on home plate with his second attempt to make it official.
"In the exuberance of celebrating," Angels manager Mike Scioscia said, "he twisted his [left] ankle. It will change the way we celebrate. I know it's happened before. Unfortunately, it happened to us."
In the first inning, Torii Hunter -- the man hitting fourth, in front of Morales -- was struck on the left hand and taken to a hospital for X-rays that were negative for fractures. The center fielder is day-to-day with a bruise on the top of the hand.
"I'm OK," Hunter said, his hand bandaged. "I'll be back. I'm really worried about Kendry Morales now. Hopefully it's not bad. It's a blow when you lose him for a couple days, never mind a couple of weeks. Right now, he's our best hitter, an All-Star.
"It's crazy. I was in [the clubhouse] watching on TV, jumping up and down with ice on my wrist. Before I know it, there's silence. You hate to see a fallen teammate go down in a great moment. You can go down any time."
Morales' drive to center ended what had been a superb duel between staff aces Felix Hernandez of the Mariners and Jered Weaver.
Bobby Abreu's eighth-inning solo homer to center brought Los Angeles even, Seattle having produced an unearned run in the fourth inning on a throwing error by shortstop Erick Aybar.
"It was a good pitch, so you have to tip your hat," Hernandez said of Abreu's homer, his sixth of the season. "It was [a] sinker down and away, and he put a good swing on it."
Hernandez, who is 2-4 in 11 starts with some of the best stuff in the business, called it his "best game."
Weaver, who has faced Hernandez eight times with the Angels, winning six, wouldn't argue that point after the big right-hander carried a five-hit shutout into the eighth.
"Felix was on his game," Weaver said. "Bobby came up big with the home run to get us in extras."
An error by second baseman Chone Figgins, the popular former Angels third baseman, preceded Morales' homer. With one out against Brandon League, Maicer Izturis doubled to the right-center gap and Abreu was walked intentionally.
When Figgins booted Reggie Willits' grounder, the bases were loaded for Morales, who lifted one of his towering drives that carried over the wall to the left of center.
It was Morales' club-high 11th homer, giving him a team-best 39 RBIs. It was the Angels' eighth all-time walk-off grand slam and the first since Hunter unloaded on April 7, 2008.
"Our 4-5 hitters both went down," said Weaver, who yielded only three hits and three walks while striking out six in seven strong innings. "Any time the meat of your lineup goes down on the same day, you can't help being discouraged. Those guys have been solid for years, and for years to come.
"It was just a fluke thing. You never want to see something like that happen to a guy having a great season. Hopefully it's not as serious as we think, and he'll be back."
Hunter saw Morales jump twice, missing the plate the first time and then leaping awkwardly, off the wrong foot, when he came down on the ankle, landing hard in the batter's box.
"He went up in a non-athletic way the second time and fell backwards," Hunter said. "He was off-balance for the second jump.
"I always said somebody was going to land on somebody's foot, twist an ankle, spike somebody in one of those celebrations. I've thought about that for years."
Willits, scoring in front of Morales, was on the periphery of the semi-circle of Angels waiting to celebrate with the game's hero.
"I've never seen anything like that," Willits said. "I don't think anybody's ever seen anything like that. I just hope he's all right. It was just a crazy deal. I'm kind of at a loss for words.
"He looked like he was in pain. I'm hoping it just scared him. I know I sprained an ankle pretty bad in high school. You hear that pop and it scares you to death."
Morales has started all 51 games for the Angels at first base, a position where they don't have a great deal of depth. Robb Quinlan is available at Triple-A Salt Lake, and Howard Kendrick and Michael Ryan have played first as well as catcher Bobby Wilson, who went the distance behind the plate on Saturday.
Brandon Wood, on the 15-day disabled list with a right hip flexor strain, also has experience at first. Mark Trumbo, one of the club's power prospects at Salt Lake, is another possibility.
A veteran left-handed hitter, former Padres outfielder and first baseman Paul McAnulty, has been enjoying a big season at Arkansas, the Angels' Double-A affiliate. He's batting .331 average with 14 homers and 27 RBIs in 44 games.