De Leon (elbow) threw 10-15 pitches in a simulated game Monday, Juan Toribio of MLB.com reports.
Toribio adds that De Leon remains over a month away from returning to game action at the big-league level, but Monday's level of activity represented the latest step in his path back from Tommy John surgery. The right-hander, who's just over a year removed from the procedure, had been building up his arm strength in batting practice sessions prior to Monday.