We’re back with five more projections, now that those nifty percentile forecasts have been released.
Baseball projection systems aren’t a toy, but they also aren’t gospel or a guarantee that certain things will happen for sure. They’re a helpful guess, albeit a sophisticated one, at what could go on with a team or a player or the league as a whole. This mostly means that projections tend to need a little more explanation than whatever stat line they spit out, and that’s our goal here today: we’ll look at five Red Sox projections from Baseball Prospectus’ PECOTA system that stood out, whether that was for good or bad reasons.
If you’re feeling deja vu, yes, we already hit on five Red Sox PECOTA projections that intrigued us, and introduced them the same way. The difference now is that PECOTA’s percentile forecasts are out, giving us a whole different level to analyze.

In short, your standard projection is a weighted mean — it’s the average of a range of projections, from best-case to worst-case, which attempts to give you the most-likely outcome. There is plenty to learn from what PECOTA believes the best- and worst-case scenarios are for a player, though, so that’s what we’ll delve into this time.
Mookie Betts’ greatest would be incredible
PECOTA was a big fan of Mookie Betts, even for his standard projection, which put him at .284/.360/.422 with a .291 True Average (TAv, Baseball Prospectus’ version of wOBA, except on a batting average scale where .260 is always average thanks to math). It’s not surprising, then, that his best-case 90th percentile forecast sees him as a star who would easily win the AL Rookie of the Year award if he were only eligible. He’s projected for a .325/.407/.485 line and .329 TAv, and over 700 plate appearances would be worth roughly seven wins — and that’s without PECOTA thinking he’s more than an average defensive outfielder.

Photo credit: Kim Klement-USA TODAY Sports
It’s an unlikely 2015 outcome for Betts, sure, but it’s still heartening to see that even his 10th percentile — or worst-case — forecast has him hitting .236/.306/.350, which is below-average but well above replacement level. If the worst Betts is expected to do is struggle at 22 the same way Xander Bogaerts did last summer, that’s not bad at all, especially when you consider it’s in the range of least-likely outcomes.
Xander Bogaerts’ breakout is still in the cards
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