"He doesn't need to be in the Home Run Derby," Mattingly said before Monday's 13-4 win over the Padres. "He needs to get a couple of hits before he gets in the Home Run Derby. He hasn't been swinging that good lately."
Fernandez, 23, has been a pretty solid hitting pitcher, posting a .187 career batting average with 20 hits. He has two career home runs, along with two doubles and a triple.
This season, he's not been as successful, sporting a .174 (4-for-23) average.
MLB, according to ESPN, has considered the idea of a pitchers' version of the Home Run Derby.
One idea that has been tossed around is matching Madison Bumgarner of the Giants against Noah Syndergaard of the Mets. Both have tremendous power.
Fernandez has showed some power himself.
Mattingly doesn't want to see it on display in an event for pitchers with a national television audience and a packed ballpark watching.
"It would be something I wouldn't be comfortable with," Mattingly said. "I wouldn't be comfortable with it. That's just my guy."
Fernandez is two years removed from Tommy John surgery, and he's on an innings range of around 180. The Marlins don't want anything extracurricular that may cause a freak injury.
"I'm comfortable with Bumgarner doing it," Mattingly joked. "He's not my guy."
• Marlins prospect Stone Garrett, an outfielder at Class A Greensboro, is expected to miss about six weeks after undergoing surgery to repair a torn tendon in his right thumb. Garrett was involved in a freakish injury that involved his teammate, Josh Naylor, Miami's first-round pick in 2015. Naylor was pulling off a prank with a knife and accidently sliced Garrett's thumb. Naylor is serving a suspension and is working with the extended spring Marlins in Jupiter, Fla.
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.