Bob Welch, a pivotal member of the A’s World Series teams of 1988, 1989 and 1990, has died at 57, the team announced today.

The cause of death was unavailable, the A’s said.

“I’m just speechless, heartbroken and sad,” Dave Stewart said of his friend and teammate.

Welch pitched for the A’s through the end of his playing career in 1994, winning 96 of his 211 games, including 27 in 1990, the year he won the Cy Young Award. Since then, no pitcher has won as many as 25 games.

Welch pitched his first 10 seasons for the Dodgers, one of his most memorable moments coming in the 1978 World Series when, as a 21-year-old, he struck out Reggie Jackson of the Yankees with two runners on base to end Game 2.

The A’s acquired Welch in a three-team trade on Dec. 11, 1987, in which they dealt Alfredo Griffin and Jay Howell to the Dodgers. With the A’s, Welch and Stewart led a dynamic pitching staff that dominated the American League for much of the late ‘80s and early ‘90s.

“This is a sad day for the entire A’s organization,” said A’s general manager Billy Beane, who played with Welch in Oakland. “Those of us who knew Bob as a teammate and a friend will miss him greatly. My condolences go out to his family.”

A’s president Mike Crowley said, “We are deeply saddened by the sudden passing of Bob Welch. He was a legendary pitcher who enjoyed many of his best seasons with the Oakland A’s. He will always be a significant part of our franchise’s history, and we mourn his loss. We send our greatest sympathies to his family and friends.”

Welch was on two championship teams, the 1981 Dodgers and 1989 A’s. He didn’t pitch in the 1989 World Series, which was interrupted because of the Loma Prieta earthquake. Only Stewart and Mike Moore started the four-game sweep of the Giants. The earthquake severely damaged Welch’s home in the Marina.

Stewart knew Welch dating to their Dodger days. They were workout partners, and Stewart noted Welch was a super golfer.

“He always had kind words to say about anybody,” Stewart said. “The most important thing about Bobby is he’s just a great friend to everybody, whether he knew you for years or just met him yesterday.”

Hall of Fame closer Dennis Eckersley said Welch’s death “hit me like a ton of bricks. I was so affected by him. We had a special connection, both of us being alcoholics.”

Two years before he joined the A’s, Welch chronicled his struggles with alcoholism and recovery in a book he co-authored: Five O’Clock Comes Early: A Ballplayer’s Battle With Alcoholism, published in 1981.

Eckersley revealed his own bout with alcoholism in a Sports Illustrated article after the 1988 season, Welch’s first in Oakland.

“I was excited to meet him. At that time, I just started my recovery. I was sober for a year when I met him,” Eckersley said. “He came at the right time in my life. Looking back, he was brave to write that book, man. He touched a lot of people, helped a lot of people. He leaves his mark with that book.”

Though older by seven years, Dusty Baker had a tight relationship with Welch and his family beginning when they were Dodger teammates. Baker called Welch in recent days about hooking up in Arizona, where Baker’s son, Darren, would play in a tournament.

“I knew him from the time he was a little kid with the Dodgers. He was like my son at first, then grew into my brother,” Baker said. “All he wanted to do was be involved in baseball, but he was much more than a ballplayer. The most genuine dude I’ve ever met. I love him, man. It hurts.”

Welch was the Diamondbacks’ pitching coach in Arizona’s 2001 championship season and most recently served as an A’s special instructor. He worked in spring training and last month was in uniform at the Coliseum, assisting the coaching staff. He has helped A’s prospects, including catcher Bruce Maxwell, now at Class A Stockton.

“I got to know him, and he was just a nice, down-to-earth guy, and he was always helping people,” Maxwell said. “It didn’t matter if you were a pitcher or an infielder. He always acknowledged the catchers — if a pitcher had a good game, Welchy always came to us first and said, ‘Hey, way to go.’

“He was always bouncing around, spreading good vibes. And when he spoke, you listened — he’d been there, done it. He was a big name in baseball. We were lucky to have him there.”

Pitcher Seth Frankoff of Double-A Midland also was touched by Welch.

“All of us on the minor-league side are very fortunate to have had Bob around the last two years,” Frankoff said. “He made a profound impact on all of us, and he truly cared about each and every one of us. He wanted us all to succeed.

“Welchy was always upbeat and positive. He’d always come up and ask, ‘Are you chukking it today?’ He was always there to confide in and ask about anything. He wanted to know about whatever you were dealing with on or off the field, and he’d had his fair share of off-field stuff and always wanted to help. He wanted all of us to be our best, and he was especially good at the mental side of the game — go after guys, be aggressive.

“He helped me tremendously. I can’t say enough about him. You hate it for his family — as a person, he meant so much to so many people. He’s someone everyone loved to be around. This is a loss for everyone. We all feel so deeply for his family.”

Welch made an impact with stadium employees and clubhouse workers. In fact, Mike Thalblum named his first son Stewart Robert Thalblum after Stewart and Welch.

“Everyone loved Bob,” Thalblum said. “As many demons as Bob had, he has as big a heart as anyone I’ve ever met. . . . He taught me to do something for everyone every day without expecting something in return, and that’s how I’ve tried to live my life. He took people under his wing and did the nicest things for them. This guy was so loved and respected, not just as a player but as a person. And what he went through to get on the field — he was second to none in his competitiveness, the only other guy I can even compare to him is Tim Hudson.”

Long-time A’s clubhouse manager Steve Vucinich echoed the thoughts.

“From Day One, everyone at my level in the clubhouse, all the clubhouse staff, he treated us all like personal friends, not like co-workers,” Vucinich said. “And it was long-lasting friendships and communication, even after he was done playing. And what a competitor. My first recollection of him is when he was with the Dodgers, facing Reggie in the World Series.

“He was as down to earth as anyone I’ve ever met. He was a little out there, and it was beautiful — he could laugh and joke around with anyone.”

Welch’s son, Riley, was drafted by the A’s in the 34th round in 2008.

Welch is survived by sons Dylan, 25, and Riley, 23, daughter Kelly, 18, and his former wife Mary Ellen. Memorial services are pending.

Staff writer Susan Slusser contributed to this report.