Michael Weiner, the union chief of the MLBPA, is undergoing treatment for a brain tumor.

He began treatment Monday, and the union said the treatments are expected to last about one month. The union anticipates he will continue to work from its New York office on a daily basis during the treatments.

Weiner, 50, succeeded Donald Fehr in December 2009 to become just the fourth head of the union since 1966.