Jose Fernandez stood on the mound Tuesday preparing to throw his first pitch of the spring when his catcher, fellow minor leaguer Wilfredo Gimenez, suddenly stood up and walked away. Gimenez had been shooed aside by Rob Brantly, the Marlins’ starting catcher, who crouched behind the plate and nodded for Fernandez to let it fly. “I just kind of snuck on over there, Brantly said with a grin. “I wanted to see Fernandez today.’’ On the first day of workouts for Marlins pitchers and catchers, the biggest buzz was about a 20-year-old right-hander who probably won’t make the opening day roster. After going a combined 14-2 with a 2.02 ERA over his first two minor league seasons, Fernandez is one of the top prospects in baseball. He is in line to open the season at Class AA Jacksonville, but all signs point to him joining the Marlins some time in 2013. “He was (Miami’s 2012 minor league) pitcher of the year for a reason,’’ Brantly said after catching Fernandez’s first bullpen session. “I think 99 percent of the pitches he threw were at my knees or right below it, where you want to be. He came up there with good poise, good mound presence. He looks like a big-leaguer.’’ He is wearing No. 78 this spring — numbers not usually seen on a major leaguer’s uniform — but might be pitching at Marlins Park by the All-Star break. Miami will open the season with a rotation that will include veteran Ricky Nolasco and three young right-handers: Jacob Turner, Nathan Eovaldi and Henderson Alvarez, who was acquired from Toronto. The fifth spot is up for grabs, most likely going to a non-roster invitee with major league experience.