Manager Bud Black also indicated that left-handed pitcher Juan Oramas will be optioned to the team's Triple-A roster on Monday.
When Oramas departs, the team will have 56 players left in camp, meaning there will be more cuts moving forward, possibly as soon as Monday.
Asencio had one hit in five at-bats and will being his season with Double-A San Antonio. Ross impressed during camp. The younger brother of Padres pitcher Tyson Ross, Joe Ross allowed one run in two games over three innings.
"Joe did fine. For a 20-year-old, in his first big league camp, I thought he presented himself very well," Black said of Ross, who will begin the season with Class A Lake Elsinore. "I thought he threw the ball well."
Portillo allowed three runs in three innings in three games. He's being converted from a starter to a reliever, a conversion that will continue in the Minor Leagues. Robertson was hitless in eight at-bats. He'll begin the regular season at Triple-A El Paso.
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.