The 41-year-old Clark joined the union staff as director of player relations in March 2010 following a 15-year big league career. Easily recognizable at 6-foot-8, Clark assisted in collective bargaining the following year and was most recently director of player services.
"Michael is my mentor," Clark said in a telephone interview. "He was and is the guy that I lean on for advice personally and professionally. In coming on board, all I had wanted to be was an extension of him in whatever capacity I could be. That's not going to change."
Weiner took over from Donald Fehr in December 2009 to become the fourth head of the union since 1966. Weiner said last August that he had been diagnosed with an inoperable brain tumour and said last week his symptoms increased in June. He must use a wheelchair and cannot move his right side or right arm.
"Tony's rise within the union will come as no surprise to those who know him," Weiner said in a statement. "It was clear from the moment Tony joined the MLBPA that his on-field experience and passion for the fraternity of players would make him a tremendous advocate for all who play the game."
Clark's appointment was approved 26-0 by the union's executive board, which will address the situation during its annual meeting in December at La Jolla, Calif. If Weiner can't serve between now and the board meeting, Clark would become interim executive director.
"I came aboard a few years ago in large part because Michael shared with me what his vision was and is for the players' association going forward," Clark said. "In my role as director of player services, in my role now as deputy executive director, that vision and my excitement in being a part of it doesn't change."