With Spring Training approaching and pitchers and catchers beginning their workouts on Feb. 13 (with position players following on Feb. 18), MLB.com is posting a series of position-by-position breakdowns leading into camp. Up next: the infield.
MIAMI -- Perhaps the area on the Marlins' roster with the most certainty is the infield, but it's also a unit that may require the most flexibility.
When Spring Training gets underway next month, manager Don Mattingly will have a pretty good idea of his options for all four infield spots. He just may be rotating players in and out -- sometimes on a daily basis -- getting the right matchups.
MLB.com breaks down the biggest questions and leading candidates to secure the infield spots.
Still up in the air: Who plays first?
First base was the biggest question entering the offseason for Miami, and it remains unsettled. There's still time to make a roster move, such as a free-agent signing or an addition via a trade.
Until then, Peter O'Brien is the leading candidate to handle the position when Spring Training opens. A Miami native, and right-handed-hitting power threat, O'Brien made a strong impression as a September callup in 2018, belting four home runs and driving in 10 runs, while hitting .273/.338/.530.

O'Brien also checks off some boxes in terms of hard contact rates. According to Statcast™, his average exit velocity of balls put in play is 92.1 mph, and his average launch angle is 16.5 degrees. Both figures are well above the league average.
On the flip side, the 28-year-old had not appeared in the Majors since 2016 with the D-backs, and he's struggled throughout his professional career making consistent contact.
Garrett Cooper, who was the Opening Day starter in right field for the Marlins last year, appeared in just 14 games all season due to a right wrist injury, which led to surgery. Cooper took batting practice on the field this week at the organization's hitters camp and will also get a shot to play first base regularly.
Is Castro on the trading block?
Barring being dealt before camp opens, Starlin Castro is expected back at second base. The four-time All-Star had a productive 2018 in his first season with the organization, posting a slash line of .278/.329/.400, with 12 home runs, 32 doubles and 54 RBIs.
The Marlins are open to dealing Castro, who is in the final season of his contract with a $16 million team option looming in 2020 (along with a $1 million buyout). But his $11 million salary for 2019 has reduced his trade value. A more realistic timetable to move Castro is around the July 31 non-waiver Trade Deadline.
Splitting time at shortstop?
Miguel Rojas made a case to be the everyday shortstop last year, appearing in a career-high 153 games. But it's likely Rojas will once again share the position with JT Riddle, who is projected to be the lone left-handed-hitting regular currently on the roster.
Rojas is also the most versatile player on the roster, capable of being a plus defender at shortstop, third, second and first base. Rojas started 78 games at short last year, and he was also the primary late-inning defensive replacement at first base.

Riddle opened 2018 on the disabled list as he recovered from the right shoulder surgery that he underwent in 2017. The 27-year-old appeared in 102 games last year and had his ups and downs, batting .231/.277/.377 with nine home runs, 10 doubles, four triples and 36 RBIs. This is a big season for Riddle to see if he can be counted on as a productive regular.
Stability at third base
Brian Anderson is generally regarded as the third baseman of the future, yet the 25-year-old might wind up in right field. A year ago, Anderson played 71 games at third base, compared to 91 in right field. The status of Martin Prado, and how the outfield shapes up, will determine what position Anderson mostly plays.
Due to hamstring, knee and oblique injuries, Prado played in just 54 games a year ago. If he is healthy, the veteran could handle third base more regularly, meaning Anderson could wind up in right. Foremost, Prado has to establish health.
Prospect watch
Isan Diaz is getting closer to being big league ready, and he perhaps could take over at second base around June or July. Ranked by MLB Pipeline as Miami's No. 9 prospect, Diaz is a left-handed hitter who spent last season at Double-A Jacksonville and Triple-A New Orleans, where at age 22, he combined to hit .232 with 13 home runs and 56 RBIs.
Diaz projects to start off 2019 at New Orleans.