Díaz played in Major League Baseball (MLB) as a catcher for the Cleveland Indians, Texas Rangers, Montreal Expos, St. Louis Cardinals, and Los Angeles Dodgers from 1996 through 2006.

Following the 2002 season, he was traded to the Texas Rangers to replace All-Star catcher Iván Rodríguez, who left for free agency.

He continued his career as a backup catcher with the Montreal Expos in 2004, the St. Louis Cardinals in 2005, and the minor league Buffalo Bisons in 2006.

At the conclusion of the 2006 season, Díaz signed a minor league contract with the Pittsburgh Pirates and was assigned to their Indianapolis Triple-A club at the end of 2007 spring training.

On June 30, 2007, while scoring from third on a Brad Eldred multiple RBI hit, Díaz suffered a total rupture of his patellar tendon when he crossed home plate, forcing him to miss the remainder of the season.