American League President Lee MacPhail has agreed to become president of the Major League Baseball Player Relations Committee, it was announced yesterday by committee chairman Dan Galbreath of the Pittsburgh Pirates.

MacPhail succeeds Ray Grebey, who resigned April 7.

Galbreath said MacPhail will assume his labor relations duties immediately and will continue as AL president through the end of the season. He has been the league's president since 1974.

Barry Rona, the committee counsel who became acting director when Grebey resigned, will continue as general counsel.

Besides Galbreath and MacPhail, other members of the PRC board of directors are National League President Chub Feeney; Joe Burke, president of the Kansas City Royals; Eddie Chiles, chairman-president of the Texas Rangers; Bob Howsam, vice chairman of the Cincinnati Reds; John McHale, president of the Montreal Expos, and Bud Selig, president of the Milwaukee Brewers . . .

J.R. Richard, who suffered a near-fatal stroke three years ago, was assigned to the Houston Astros' Gulf Coast Rookie League in Sarasota, Fla.

The assignment was seen as another step in Richard's major league rehabilitation program. Astros officials said Richard was not on a timetable for either his first appearance in Sarasota or advancement through the club's minor league system.