The unexpected move comes because of injuries to right-handers Nathan Eovaldi and Henderson Alvarez. They had been announced as part of the rotation, but instead both will start the season on the disabled list because of shoulder inflammation.
Also, first baseman Logan Morrison was placed on the 60-day disabled list while he recovers from knee surgery last September, meaning he won't rejoin the lineup until June.
Fernandez was the Marlins' first-round draft pick in 2011 and hasn't pitched above Single-A. The 20-year-old Fernandez was part of the Marlins' spring training camp this year but was then sent to Double-A Jacksonville and hadn't been projected to reach the majors until June.
Ricky Nolasco will start the Marlins' opener Monday at Washington. Right-hander Kevin Slowey will start the second game of the series and left-hander Wade LeBlanc will start the finale.
Eovaldi and Alvarez are expected to be sidelined for about a month.
Fernandez, a 6-foot-4, 220-pound right-hander, came to the United States by boat as a Cuban refugee in 2008. He was the 14th overall draft pick in 2011 as a high school senior in Tampa, Fla., and last year he went 14-1 with a 1.75 ERA in 25 games in the minor leagues.