The Athletics selected Alvarez's contract from Triple-A Las Vegas on Saturday.
Alvarez has been playing well in Triple-A this season, slashing .311/.401/.528 with 27 runs, eight home runs and 33 RBI over 187 plate appearances. His move to the big club corresponds with Abraham Toro being placed on the injured list due to a right hamstring strain. While Tyler Nevin figures to get most of the reps at the hot corner with Toro sidelined, Alvarez could see some playing time at third base and, in the process, make his major-league debut.