"Nothing's official at this point so nothing really to announce," Mattingly said. "We'll probably do it [Sunday]."
Mattingly cited a delay with the paperwork as reasoning for holding off the announcement and said all the players involved in the decision already know where they will begin the season.
Outfielder Tyler Moore is expected to claim that final spot. Moore hit .308 with a team-high five home runs in the spring. He also gives Miami a right-handed bat who can play first base. Moore, 30, has spent parts of four seasons in the Major Leagues, the last being in 2015 with Washington.
He missed four months of 2016 following a knee injury.
Matt den Dekker and Brandon Barnes, both non-roster invitees to camp, remained with the club through Saturday's game but are likely to begin the season at Triple-A.
Aside from naming the final player on the roster, Mattingly said the Marlins were prepared to break camp.
"We're as ready as we are going to be at this point," he said. "I feel good about our club. Looking forward to getting started."
With a 3-2 victory over Detroit in the Grapefruit League finale, Miami concluded spring play with a 13-17-5 record.
Mattingly is ready to learn whether what he saw this spring will carry over into the season.
"Obviously you look at things on paper and where things may work out, but until you actually get out there and start kind of going game to game and get some innings on the board, you don't actually know," Mattingly said. "I feel confident with our club. I trust these guys that we're ready to go and that they'll be ready."
Mattingly returned to that trust factor a couple times while meeting with the media prior to Saturday's game.
Miami is still a young club, but the core group of Giancarlo Stanton, Christian Yelich, Marcell Ozuna, Adeiny Hechavarria and J.T. Realmuto have played together for several years now.
"That's a good thing for a manager to feel like you trust your club," he said. "I can say that."