Mookie Betts, OF, Boston Red Sox
2017 fWAR: 5.4
2018 fWAR (so far): 6.5
Mookie Betts actually had a “down” season in 2017 and still managed to be worth more than five wins. That also puts into perspective just how good he’s been throughout the first half for the Boston Red Sox.
His 23 home runs are a bit off the pace of the league leaders, but his .332 ISO is the best among qualified hitters. He’s also the only hitter eligible for the batting title with a wRC+ over 200 (202, to be exact). If he can somehow sustain this pace until the end of September, he’d be the first player not named Barry Bonds to do so since the 2000 season.