Miami is off on Thursday before opening a three-game set at Citizens Bank Park on Friday.
"We're close to getting Giancarlo back," Mattingly said. "We're hoping he can play Friday. We were missing that for about 45 days and that hurt us."
Stanton has made a remarkable recovery from a Grade 3 left groin strain suffered on Aug. 13. Initially, the fear was he would be out for the rest of the season. But then the club became hopeful he could return in a pinch-hit role the final week of the season.
But Stanton has defied the odds, and was reinstated on Sept. 6, but thus far has only been used as a pinch-hitter.
Now, he could be back in right field on Friday.
For more than a week, Stanton has been moving around without showing any signs of discomfort in batting practice. He's not only hit, he's running the bases and working on fielding drills in right field.
Citizens Bank Park makes sense as a place to get back into action because there isn't as much ground to cover as Marlins Park.
Now four games back of the Mets for the second Wild Card, the Marlins are starting to get several core position players back. Justin Bour and Marcell Ozuna are at full strength, and the offense is picking up.
Adding Stanton to the middle of the order could help the club's final push. Stanton has 25 home runs and 71 RBIs.
"Since we've been able to get J.B. back and Marcell back healthy again, it's like the two-headed or three-headed monster becomes a lot more dangerous," Mattingly said. "Then you have five guys in there and you have some add-on guys and it takes the pressure off."
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.