For some-to-all of his Major League Baseball career, new St. Louis Cardinals center fielder Dexter Fowler has exhibited just about every skill you could realistically hope to have in a baseball player. In 2009, his first full season in MLB, with the Colorado Rockies, Fowler stole 27 bases, good for 6.8 FanGraphs Baserunning Runs Above Average.
While speed among MLB players tends to decline more quickly than other skills, it is worth noting that his second-best season by BsR was 2016, his age-30 season. It seems plausible that Fowler is as adept at stealing bases as he ever was—he has simply played for teams which opted not to utilize him in this fashion. Batting in front of Anthony Rizzo and Kris Bryant likely means a stronger philosophical opposition to running into outs when such a fate can be avoided.
The other baseball skill which generally declines on nearly as linear of a downward trajectory as baserunning is defense. It is particularly common for sharp defensive declines in center field, the position most reliant on speed. Given Fowler’s poor, Worst Defensive Center Fielder In Baseball Not Named Matt Kemp level metrics before beginning his Chicago Cubs tenure, it would have been reasonable to assume that Dexter Fowler would be moved to a corner outfield position by this point in his career.
But aided by a surprising defensive renaissance, the formerly atrocious defensive center fielder became, over the last two seasons, within one run of league-average by Ultimate Zone Rating. Let’s just round Fowler to average, for the sake of clarity.
And while doing this spits in the face of conventional sabermetric wisdom, a philosophy which generally assumes that one needs at least three years of defensive statistics to draw any real conclusions (and in 2014, with the Houston Astros, Fowler had his worst defensive season), there is some reason to believe that Fowler’s newfound defensive adequacy is a sincere reflection of his potential going forward—Statcast data shows that Fowler played the deepest center field defense of his career in 2016, and perhaps not coincidentally, he reached his statistical peak.
The average outcome is probably that Fowler is a slightly below-average center fielder, capable of sticking at the position but hardly a Gold Glove candidate, but he has demonstrated at least some reason to believe that he could be an average fielder.
Dexter Fowler has never been a notable power hitter, but his power has generally increased over time. He hit just 15 total home runs over his first three seasons, followed by 13 and 12 dinger years to close out his Rockies tenure. His total dipped to eight in 2014 (considering he left Coors Field, this is understandable), but jumped back up to 17 and 13 over the last two seasons. His home run total could go down by virtue of playing at Busch Stadium, a less homer-friendly park than Wrigley Field, but even considering park factors, Fowler does seem far removed from his single-digit home run seasons. ZiPS has him at 11 and Steamer has him at 12 for 2017, but a realistic but still somewhat sunny target might be 15, the midpoint of his last two seasons.
The most predictably strong facet of Fowler's game throughout his MLB career has been his ability to draw walks. His single-season career-low walk rate of 11.3% is within a percentage point of the career mark of Matt Carpenter, a player noted for his plate discipline, and it has led to a career .366 on-base percentage for Fowler, which is in the top twenty among all active players. 2016 brought a career-best 14.3% walk rate, aided somewhat by being surrounded by a superb lineup. While the 2017 Cardinals lineup isn't projected to be as good as the 2016 Cubs (to be fair, neither are the 2017 Cubs), it's within the realm of possibility. Especially if the Cardinals come to their senses and bat Matt Carpenter second in the lineup, a 15% walk rate by Fowler is a non-ridiculous potential outcome.
So what will happen if Fowler is able to combine the best of his performance: plus base-running (his best season by Baseball Reference Baserunning Runs had him at +5 runs), an average center field glove, 15 home runs, and, say, 90 walks (15% of 600 plate appearances)? Sounds like a pretty good season, right? Wanna take a guess how many center fielders in baseball history have managed a season matching or exceeding this level of play?
The answer, which I gathered thanks to the indispensable Baseball Reference Play Index, is six: Mickey Mantle did it four times, and five others did it once—Mike Trout (last year), Carlos Beltran, Rickey Henderson (who only spent two seasons primarily in center field), Dale Murphy, and Willie Mays (who reputationally aged poorly, but did this at age 40).
Admittedly, this is an optimistic view of Fowler. While some of this may come together, it probably won't all come together. But one thing to keep in mind is that Fowler is coming off of his best MLB season overall and he has, to this point, aged well—expecting a 31 year old to hit his defensive peak would ordinarily be preposterous, but given the trends in his career stats, it should not take any fans completely aback.
To expect Dexter Fowler, or any free agent, to surpass his previous best performance, would be unfair, but Fowler may have as good of a case as anybody to surpass his own career precedent.