Mikolas had a 3.62 ERA in 25 appearances last season, his first in the Major Leagues.
The Padres are getting closer to determining their seven-man bullpen, though closer Huston Street, Luke Gregerson, Joe Thatcher, Brad Brach and Dale Thayer are essentially guaranteed of spots.
The Padres opened camp last month with 67 players. They don't need to finalize their 25-man roster until before Opening Day on April 1 in New York against the Mets at Citi Field.
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.