"He threw his last sim game," Mattingly said. "We think he's getting close and ready to start."
Chen, who is 5-4 with a 4.99 ERA in 19 starts, has been on the disabled list since July 25 with a left elbow sprain. In recent weeks, he has been building back up in hopes of making at least a start or two in the final three weeks.
The Marlins have not committed a specific date for Chen to come back, but one possibility is filling in for prospect Jake Esch, who will start on Tuesday at Atlanta.
The Marlins are at Atlanta through Wednesday and are off on Thursday before a weekend series at Philadelphia. The rotation is set for that series, with Tom Koehler on Friday, Jose Urena on Saturday and Andrew Cashner on Sunday.
Miami could perhaps go with Chen on Monday at home against the Nationals.
"There is another day coming up a spot start would be needed, and we probably would have the flexibility to put him in earlier, if we needed to," Mattingly said. "We're having discussions about where to drop him in."
• Giancarlo Stanton continues to go through full batting practice drills, and there is a chance in a week to 10 days that he could be ready to play right field.
Stanton was reinstated last Tuesday after being out since Aug. 14 with a Grade 3 left groin strain. His role, for now, is as a pinch-hitter. But Stanton has been practicing in the outfield, running down fly balls as well as running the bases.
"It seemed like we were talking 10 days a few days ago," Mattingly said. "We're getting within range. If you watch him, he's doing more and more out there. We'll just let medical dictate where we're at, when we're going to do it. Is it going to be a full game when he starts? Is it going to be half a game? How are we going to handle things like that?"
• A.J. Ramos collected the save in Sunday's 3-0 win over the Dodgers at Marlins Park. If a save situation arises, Ramos again will be the first choice moving forward, instead of Fernando Rodney.