The union said Tuesday that he began treatments on Monday and that they are expected to last about one month. The union said it anticipates he will continue to work from its New York office on a daily basis during the treatments.
"It's shocking," Toronto pitcher Carlos Villaneuva said. "We told him, take care of himself first, which is the most important part of it all. I know he's going to want to go into the office every day and keep track of everything.
"It's just the way his brain is wired. He was more concerned about us when we had our discussion about it. There were maybe 100 players on that phone call, and nobody said a word. We were all shocked. But he never buries the lead. He came right out with it."
The 50-year-old Weiner succeeded Fehr in December 2009 to become just the fourth head of the union since 1966. He is widely liked and respected among players and management, and he has been credited for an improved relationship between sides.
"I have great respect and admiration for Michael, with whom we have had a very constructive relationship both professionally and personally," baseball Commissioner Bud Selig said in a statement. "This relationship has been a great benefit to baseball and has led to the tremendous success the game now enjoys. All of us look forward to Michael's full recovery and to his continued contributions to our game."
Following his graduation from Williams College and Harvard Law School, Weiner clerked for a federal judge and was hired by Fehr in 1988 as a union staff lawyer.
Under Weiner's watch, the union signed an agreement in November for a five-year contract running until December 2016, which ensures 21 consecutive years of labour peace in Major League Baseball. The agreement allowed for blood testing for human growth hormone, introduced restraints on bonuses for amateur draft picks and international signings, and restored salary arbitration eligibility for part of a class of players that lost it in the 1980s.
"He's ultra-prepared and smart," Chicago White Sox player representative Paul Konerko said. "I mean I think everybody is happy with the last agreement that got done."
Under Weiner, the union also succeeded in a grievance overturning a positive drug test against NL MVP Ryan Braun, who avoided a 50-game suspension.
"He's upbeat about everything and he's looking forward to hopefully beating this," San Francisco Giants pitcher Matt Cain said. "Obviously, it's going to be a process. We hope he can keep his strength and motivation up to make it through it."
AP Sports Writers Rick Gano and Noah Trister contributed to this report.