Consider it insurance in case the team gets in a bind and needs someone to man the hot corner in a pinch.
"We've got a third baseman who is day to day with a biceps strain, and if something happens tonight, we might need someone to go over there," Black said. "[Alonso] has got good hands and a good arm. We're going to look at it a little."
Alonso has appeared in two games at third base during his Major League career -- with the Reds in 2011 and once with the Padres last season.
Alonso isn't a complete stranger to the position. He played it a lot as a youth and moved over to first base at the University of Miami because Ryan Braun played third base.
Corey Brock is a reporter for MLB.com. Keep track of @FollowThePadres on Twitter. This story was not subject to the approval of Major League Baseball or its clubs.