NEW YORK -- Michael Weiner, the plain-speaking, ever-positive labor lawyer who took over as head of the powerful baseball players' union four years ago and smoothed its perennially contentious relationship with management, died Thursday, 15 months after announcing he had been diagnosed with an inoperable brain tumor. He was 51.

The Major League Baseball Players Association said Weiner died at his home in Mansfield Township, N.J.

"Michael Weiner worked even thru his sickness. He didn't look at it as an excuse to quit," tweeted Pittsburgh's Andrew McCutchen, the NL MVP. "He never gave up on us even when at his worst."

As Weiner's health deteriorated this summer, a succession plan was put in place. Former big league All-Star Tony Clark took over Thursday as acting executive director and is to be approved as Weiner's successor when the union's board meets from Dec. 2-5 at La Jolla, Calif.

"Words cannot describe the love and affection that the players have for Michael, nor can they describe the level of sadness we feel today," Clark said in a statement. "Not only has the game lost one of its most important and influential leaders in this generation, all involved in the game have lost a true friend."

Baseball Commissioner Bud Selig called Weiner "a gentleman, a family man, and an extraordinarily talented professional who earned the trust of his membership and his peers."

"Our strong professional relationship was built on a foundation of respect and a shared commitment to finding fair solutions for our industry. I appreciated Michael's tireless, thoughtful leadership of the players and his pivotal role in the prosperous state of baseball today," Selig said in a statement. "Michael was a courageous human being, and the final year of his remarkable life inspired so many people in our profession."

At Weiner's last public speaking engagement, a 25-minute meeting with baseball writers on the day of the All-Star game in July, he was confined to a wheelchair and unable to move his right side. Yet, he wanted to respond to questions about his illness and issues in the game, and did so with the grace and humor he was known for throughout his life.

Baseball players' union leader Michael Weiner died Thursday, 15 months after being diagnosed with an inoperable brain tumor. He was 51. Patrick McDermott/Getty Images

"I don't know if I look at things differently. Maybe they just became more important to me and more conscious to me going forward," he said. "As corny as this sounds, I get up in the morning and I feel I'm going to live each day as it comes. I don't take any day for granted. I don't take the next morning for granted. What I look for each day is beauty, meaning and joy, and if I can find beauty, meaning and joy, that's a good day."

Weiner first experienced weakness and tingling on his right side in July 2012 and was diagnosed with a glioma the following month. By June 2013, he had experienced a rapid increase in symptoms. As he sat in a wheelchair in foul territory at Citi Field the following month before the All-Star game, players lined up to speak with him.

His voice had gotten raspy by early August, when he responded on behalf of the union to drug suspensions handed down to Alex Rodriguez, Ryan Braun and other players.

"We wouldn't be where we are today without his expertise," San Francisco Giants pitcher Jeremy Affleldt said in a text to The Associated Press. "We will all feel this loss of such a great man."

Known for wearing blue jeans and Converse Chuck Taylor All-Star sneakers to work, Weiner's easygoing manner with players was a change from former head Donald Fehr's more lawyerly approach. His style connected both with players and the students he taught during Sunday school at his synagogue.

Arizona Diamondbacks reliever Brad Ziegler took to Twitter to express his sympathy:

Lost a great friend today. Michael Weiner's body finally succumbed to cancer. One of the best leaders & men I knew. Prayers for his family. — Brad Ziegler (@BradZiegler) November 22, 2013

Weiner was hired by the union as a staff attorney in 1988 and wound up succeeding Fehr in December 2009. Weiner became just the fourth head of the organization since 1966.

"Here you had an individual who came to me as a kid for his first private sector job," Fehr said in a telephone interview with the AP. "He impressed me at the time and ever after with his intelligence, his dedication, his innate sense of fairness, his focus on finding what the right thing was to do and then doing it. This was an extraordinary individual all the way around."

A longtime New Jersey resident and a graduate of Williams College and Harvard Law School, Weiner clerked for U.S. District Judge H. Lee Sarokin in Newark before joining the players' association. Once at the union, he became a key figure in the lengthy process to parse the $280 million collusion settlement among individual players.

Weiner also was a junior lawyer during the 7½-month players' strike in 1994-95 strike and the negotiations that finally led to a new labor agreement in March 1997.

"I think that helped some people on the owners' side to finally accept that the union was a fixture and the union was an entity they were going to have to deal with," he said. "There was never a chance for anything to settle in until we got through collusion, and really until then we got through the bargaining in '94 and '95."

Following eight work stoppages in a 23-year span, baseball has since negotiated three straight labor deals without interruption.

Weiner headed talks for the last deal, in November 2011, which instituted a series of significant changes that included restraints on signing bonuses for amateur players and increased the number of free agents able to switch teams without requiring the loss of draft picks as compensation.

"It took a while for the owners to appreciate that the union is not only here to stay, but that the union and its members can contribute positively to a discussion about the game -- about its economics, about the nature of the competition, about how it's marketed in every way," he said.

In addition to the labor contract, he headed the legal team that in 2012 convinced an arbitrator to overturn a 50-game suspension imposed on Braun, the Milwaukee outfielder who was the previous year's NL MVP. The union argued his urine sample had not been handled properly.

Last summer Braun agreed to accept a 65-game suspension for his activities relating to the Biogenesis of America anti-aging clinic and his public statements.

Following a line of leaders that began with Marvin Miller and went on to include the short reign of Kenneth Moffett and the long tenure of Fehr, Weiner was exceedingly conscious of the union's history and traditions of player involvement. He appeared with Fehr and the then 95-year-old Miller at a 2012 discussion at New York University's School of Law marking the 40th anniversary of the first baseball strike and the rise of the union.

His hair nearly gone from his treatment, Weiner returned to NYU in January for a memorial celebrating the life of Miller, who died two months earlier. He humbly referred to "our little sport of baseball."

"He was truly a great individual, a brilliant lawyer and a thoroughly decent person," Los Angeles Dodgers President Stan Kasten said in a statement. "Michael was always viewed as the path to a reasonable resolution. He will be missed."

Weiner is survived by his wife, the former Diane Margolin, and daughters Margie, Grace and Sally. Funeral arrangements were pending.