Rodney, 39, joined his new team on Friday, a day after San Diego traded him for a minor league pitcher. The Marlins view Rodney as adding strength to an already solid group of relievers.
Mattingly will occasionally give him a chance to close, but the job still belongs to Ramos.
"Talking with him, he was great in giving him our reasoning," Mattingly said. "Like I told him, if he was the guy here and had not blown one, I would do the same thing for him. I think he totally understands."
Rodney's 253 career saves rank fifth among active pitchers. He has converted all 17 of his chances this year, and his 0.31 ERA leads the majors among qualifying relievers.
Rodney was all smiles as he spoke with reporters before Friday's game against the Atlanta Braves. He playfully seemed more concerned about having to conform to Mattingly's rule banning facial hair and shave his beard.
He was still holding out hope it won't have to go.
"If that's the rule they have here, I like to play baseball and have fun," Rodney said. "I'm not worried about shaving. I want to have fun with my teammates, play good, play hard. That's it."
Ramos has been Miami's closer since May 2015. His 24 saves rank second in the majors this season, and he has converted 33 straight chances since last September.
"With A.J. and what he's been able to do -- he's been perfect -- it would be hard for me to take that out of our mix," Mattingly said. "Our back end has been good. Fernando makes us better."
Mattingly told Rodney that he will get some chances to close when Ramos has worked consecutive days.
"Fernando has a track record obviously of being a big-time guy," Mattingly said. "He's been there, done that."
Mattingly indicated that Ramos, Phelps and Barraclough welcomed the news of Rodney's addition.
"I think these guys, for the most part, like knowing when they're going to pitch and in what situations," Mattingly said. "We can't always give them exactly when every time, but I think they like to know when the phone rings, who's getting up for the most part."
The Marlins are Rodney's seventh team and sixth since 2011, but Mattingly believes he will fit in easily.
"All the people we talked to love this guy," Mattingly said. "As a teammate on and off the field, he wants to win. Totally professional. I'm excited about having this guy. I think he's going to help our club."
Though Mattingly said the beard is "coming off," the Marlins will let Rodney keep wearing his cap cocked sideways. The "arrow," a celebratory stance he uses after earning a save, is another Rodney staple that stays.
"I'm going to save the arrow for now," Rodney said. "If you have a chance, you're going to see a landing."