PORT ST. LUCIE, Fla. -- It has been a year and a half since Amed Rosario broke into the big leagues, debuting as the second-ranked prospect in baseball. As happens with young players, Rosario struggled a bit at age 21 as he adjusted to the big leagues. So it goes. He entered the following year as the Mets' unquestioned starting shortstop, but as late as Aug. 9, he was batting .230 with a .619 OPS.

The following night, Rosario rapped out three hits, sparking a hot streak that would last until the end of the season. From Aug. 10 on, Rosario slashed .303/.335/.444 with nine doubles, two triples, five home runs and 13 stolen bases in 47 games. He carried it into this spring, recording multiple hits in two of his first three games and keeping his average well above .300 for most of the Grapefruit League season.

The question is what comes next: ascension to stardom, or stagnation as he enters what should be the prime of his career?

"I have a lot of confidence going forward," Rosario said through an interpreter. "I feel like I'm better. I feel like I've been putting in a lot of work. That's really all it is for me."

It shouldn’t take long for the Mets to discover if last year's surge was a seven-week hot streak or a harbinger of genuine improvement. In a lineup featuring Robinson Cano, Michael Conforto, Brandon Nimmo and other sluggers, Rosario does not need to be a focal point of the offense. But he could make the difference between a strong lineup and an elite one.

"We protected him at times last year, and the gloves are off this year," manager Mickey Callaway said. "He's going to have to go out there and continue to do what he did last year, and understand what he did last year that made him more successful in the second half, and carry that over into a spot in the lineup [where it's] probably a little harder to accomplish that. So we're asking a lot of a young player. But if anybody can handle it, Rosie can. He comes with a passion every day. He comes to get better."

Asked about his work this spring, Rosario talks mostly about the leg kick he added back to his swing. Last year, frustrated by a lack of success in the big leagues, Rosario transformed the kick into more of a toe tap, experiencing success with that for a time. But for him, the kick is comfortable. And so the kick returned.

Callaway and hitting coach Chili Davis don't mind so long as Rosario thrives. At his best, the shortstop is capable of producing double-digit homers and 30-plus steals to go along with strong defense and elite speed -- 26th in baseball last year, according to Statcast's Sprint Speed metric. Baseball Prospectus' PECOTA projection system predicts 13 homers and 22 steals for Rosario -- the latter number dampened by a .299 on-base percentage. If Rosario can overcome his old tendencies of expanding the strike zone, an improved OBP would give him a chance to blossom into a star.

Consider this a critical summer for Rosario in another way, too. Andres Gimenez, the Mets' No. 2 prospect, is also a shortstop, and should be big league ready by this time next year. If both Rosario and Gimenez continue developing on an upward arc, it would give free-wheeling general manager Brodie Van Wagenen the type of problem he loves to have. If not? Rosario could start feeling heat from the Mets' next big thing.

"He can still be a very, very good player at the Major League level while he's developing, and we saw that in the second half," Callaway said. "He did a great job on his defense. He did a great job at the plate, laying off some pitches and putting pitches in play. He's going to have a challenge."