Marshall, who went on the DL on April 10 (retroactive to April 8) with tendinitis in his left shoulder, is eligible for activation on Tuesday. The club must still decide whether or not he could use a game or two in the Minors to see how he bounces back before bringing him back.
On Thursday, Marshall threw his first bullpen session and reported no issues. He planned to increase the count and mix up his pitches in the second session.
"My arm has been feeling great and really strong," Marshall said. "My arm strength is really good, especially in the long-toss sessions I've had and the bullpen. It's been smooth and the ball is coming out of my fingertips real nice too. We'll keep getting better and stronger. It's been on a good path so far. The trainers are doing a great job."
Marshall, who came down with shoulder fatigue during Spring Training, has made only one appearance for the Reds this season. He threw a perfect inning vs. the Nationals on April 7.
Mark Sheldon is a reporter for MLB.com. Read his blog, Mark My Word, and follow him on Twitter @m_sheldon. This story was not subject to the approval of Major League Baseball or its clubs.