Acquired from the Cardinals in the Marcell Ozuna trade, Alcantara has impressed in Spring Training, not only in performance, but also with is command and composure.
"He's throwing the ball well," Mattingly said. "I don't think we can discount that. He's doing everything we've asked, and he seems to be picking it up.
"If we feel like he's ready to compete, and that he can continue to develop at the Major League level, then he's one of those guys we're talking about."
Alcantara has a 1.13 ERA in eight Grapefruit League innings. The 22-year-old's fastball has already reached 98 mph, and he gets another test on Sunday, as he is scheduled to throw in relief against the Yankees at Roger Dean Chevrolet Stadium.
"I think we want to keep seeing him pitch," Mattingly said. "He's throwing the ball well."
The Marlins project Alcantara as a top-of-the-rotation candidate, but at age 22, there are questions as to if he is ready for the big league grind. He's been fine-tuning his mechanics with pitching coach Juan Nieves and bullpen coach Dean Treanor.
"Dean and Juan are talking about things, about keeping him over the rubber and making sure he's using his body," Mattingly said. "Things like that. He's working very hard."
The other candidates are right-handers Jacob Turner, Odrisamer Despaigne, Elieser Hernandez and Brett Graves, and lefties Adam Conley, Dillon Peters, Jarlin Garcia, Caleb Smith and Justin Nicolino.
Lefty Chris O'Grady, who previously was a starter, is competing for a bullpen spot.
Hernandez and Graves are Rule 5 picks, who either will make the club or be offered back to their previous teams, the Astros and A's, respectively. Graves also is dealing with a left oblique strain.
The Marlins are expected to carry 13 pitchers total. Along with the five starters, they're looking at two candidates who can throw multiple innings -- at least three.
Turner, Despaigne, Nicolino and Smith have handled similar roles -- spot starter/long reliever -- at the big league level.
"We still have innings that we're going through and want to give guys chances to throw the ball," Mattingly said. "We're still looking."