In 2017, the production from the Mets outfield was similar to the production from the starting pitching in that the talent was there in both cases, but injuries played a big part in reduced production.
The left fielder, Yoenis Cespedes, had an excellent slash line at .292/.352/.540, higher in all three categories than for his lifetime stats. However, his playing time was literally a half a season, at 81 games. This was due to injuries including a pair of leg hamstring issues, one in each leg that lead to long stints on the DL at different times.
In the 2016-2017 off season, Cespedes seemed to emphasize upper body workouts, further developing his already sculpted upper physique. As reported by Kristie Ackert of the New York Daily News, Cespedes has realized that his off season regimen needs to be different from last year, presumably involving more stretching exercises for his lower body.
If that results in say 130 games or even more for Cespedes at similar production rate to last year, that would be a big boost for the Mets in 2018. Cespedes just turned 32 in October, so he still should have a few good years left in him.
In right field, Michael Conforto was having a breakout season in 2017, recording a slash line of .279/.384/.555, with 20 doubles and 27 homers in just 109 games. Conforto’s season ended early, as we all know, when he suffered a dislocated shoulder and torn posterior capsule while swing at a pitch in an August game. That’s an unusual injury, and a severe one.
Fortunately, as the New York Post reported, his rehab is proceeding well and he should be able to start swinging a bat in January. It’s still a bad injury, but that latest report shows progress and could mean nearly a full season of play.
Conforto will be 25 next season, moving right into the usual prime baseball ages. He not only hits but is a plus defensively, Fangraphs shows his UZR at a cumulative 15.5 over his three seasons in the big leagues, so he has saved some runs with his glove. If he can start the season, or at least close to the start of the season with the kind of production he flashed last year, he would be another big plus for the team.
In center field, the main in-house choices would be Juan Lagares and Brandon Nimmo. Lagares brings a great glove (golden in 2014) but spotty offense and a history of injuries. His 2017 slash line was .250/.296/.365 in 94 games. He is taking a different approach this off-season working with noted hitting savant Craig Wallenbrock. Several players in recent years have revamped their swings and shown big improvement at the plate. Notable examples would include Justin Turner, Daniel Murphy and J. D. Martinez ( a Wallenbrock pupil.) No such guarantees with Lagares but if he can improve somewhat at the plate and keep up his defense, he will be a big asset to the team.
The other center field candidate is Brandon Nimmo. In 2017 he was in 69 games and turned in a .260/.379/.418 slash line. That OBP is pretty good, and his SLG was held down by a bad June when he was coming off the DL and turned in a .286 SLG, but he clearly improved in that as the year went on.
A good approach for the Mets might be to let Lagares and Nimmo fight it out for the starting CF position and use the other player as the fourth outfielder, and if that player is Lagares, use him as a defensive replacement in the late innings of any game the Mets have a small lead in.