Sampson, a fourth-round Draft pick in 2009, began the season with Tucson but struggled there, going 0-1 with an 8.04 ERA in four starts.
Sampson was dominant with the Missions, though, especially after he began using his slider more often and with greater success.
The Padres also sent outfielder Donavan Tate, the No. 3 overall pick in the 2009 Draft, to short-season Eugene of the Northwest League.
Tate has been working out at the team's temporary Spring Training facility in Arizona since June after missing Spring Training. Tate has battled injuries and substance abuse since he was signed and joined the organization.
Corey Brock is a reporter for MLB.com. Keep track of @FollowThePadres on Twitter. Jamal Collier is an associate reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.