Chen has been on the DL since July 25 with a left elbow sprain.
"Today went very well," Mattingly said. "I stood out there with him, and the ball was coming out of his hand [well]. He looked confident throwing. He was letting it go. Obviously, we'll see how he feels tomorrow, coming out of that."
Chen, the club's Opening Day starter, is 5-4 with a 4.99 ERA in 19 starts and 110 innings.
Before putting a timeline on a possible return, Chen wants foremost to be healthy.
The Marlins are contending for a postseason spot, and having Chen for the final few weeks of the regular season could be a big boost to a rotation that also is without lefty Adam Conley (left third finger tendinitis).
"Definitely, I want to get better soon and help this team to contribute to the fight for the Wild Card spot," Chen said through his interpreter. "But since I got here, you must be patient, and you must do what you need to do to get better, and then you can help the team. You can't rush anything. You have to go through every step."
Until Chen is ready to return, Mattingly notes the club has to keep pushing forward, and if Chen returns, it will be a bonus.
"At this point, keep him on track, and don't really sit here and wait for him, because you don't really have time to wait for him," Mattingly said. "It's more, try to win a game, and just kind of keep moving, and some day you may get him, and then it's a shot in the arm."
Joe Frisaro has covered the Marlins for MLB.com since 2002. He writes a blog, called The Fish Pond. Follow him on Twitter @JoeFrisaro and listen to his podcast. This story was not subject to the approval of Major League Baseball or its clubs.