The Padres selected Diaz's contract from Triple-A El Paso on Sunday.
The 33-year-old joined the Padres on a minor-league contract Aug. 26 and will join the big club Sunday as MLB rosters expand to 28 players. Diaz may not receive consistent playing time, and he should serve as the backup to Luis Campusano on Sunday as Kyle Higashioka manages flu-like symptoms.