"There's room to improve arm strength," Glavine said. "But first and foremost, I had to be able to go out there and make my pitches without pain, and I did that."
This marked the first time that Glavine even attempted to throw off a mound since experiencing some concerning shoulder discomfort on April 12, during what had been scheduled to be his final Minor League rehab appearance.
Initially, Glavine was concerned that his troublesome shoulder had reached a point where it would no longer allow him to pitch. But since beginning to throw again last week for the first time in two weeks, the 43-year-old left-hander has steadily gained reason for encouragement.
"If I had done something structurally wrong or made something structurally wrong with my shoulder worse, I wouldn't be able to go out there and throw 150 feet," Glavine said. "I wouldn't be able to go out and throw a bullpen like I did today.
"That doesn't mean that I'm going to be pain-free or that I'm not going to have to deal with some things, but for me, my biggest concern was, 'Did I do something worse to my rotator cuff that I wouldn't be able to pitch again?' But clearly I didn't do that, because I wouldn't be able to do what I've done so far."
As long as he rebounds in positive fashion and continues to gain progress while throwing the rest of the week, Glavine will likely throw another bullpen session on Friday in Philadelphia.
If Glavine is then comfortable to take the next step, he'd like to simulate two- and three-inning outings to build up his arm strength to the point where he would have to make just one or two Minor League rehab appearances during the middle-to-latter portion of this month.
Glavine is attempting to return from an August surgical procedure, during which Dr. James Andrews repaired a torn left elbow flexor tendon and removed debris from the left shoulder.
Somewhat surprisingly, Glavine hasn't felt any elbow discomfort. While knowing that his shoulder will never again allow him to pitch in a completely pain-free manner, he was encouraged with the limited discomfort that he encountered while completing Monday's bullpen session.
"That side session was better than any that I had during Spring Training," Glavine said. "Comparing the beginning of Spring Training to the end, it was better than the beginning, but I didn't feel as strong as I did at the end. But pain, discomfort and everything like that was as good as it was at the end of Spring Training."
Mark Bowman is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.