Julien has played in every game and leads the Twins in plate appearances. Castro has a streak of complete games due only to Correa's injury. Santana... well, Rocco seems to believe in Santana, and at the moment he's getting results.
By such measure, no one on the Twins is playing full time. Julien ranks 119th in the majors in plate appearances.
Rocco and the analytics team have worked out some kind of 3-D chess strategy with their platoons, and color me skeptical. But singling out Julien or Castro or Santana obscures what's going on.