Gallen (hamstring) threw 67 pitches in a simulated game Monday and will next have a bullpen session Thursday, Steve Gilbert of MLB.com reports.
The Diamondbacks were preparing for Gallen to throw Monday, whether in a sim game or live batting practice, and decided on the sim game. He next heads to Chase Field on Thursday for a bullpen session, which will be caught by Gabriel Moreno, who is nursing a sprained left thumb. If all goes well Thursday, then Gallen could return during Arizona's series against Oakland that begins Friday.