Robertson (elbow) threw about 20 pitches in a bullpen session Sunday, Meghan Montemurro of The Athletic reports.
It's Robertson's first bullpen session since he landed on the injured list with the flexor strain in mid-April, and he still has significant work to do in building up his velocity. The 34-year-old has another session scheduled for Wednesday, and if all goes well, he could return to the Phillies toward the end of July.