Erik Miller's Primary Flaw Has Become Red Sox Pitching Lab's Newest Pet Project

There is something strange about Erik Miller's numbers. Right-handed hitters have trouble making contact against him. When they do, the ball leaves the bat at just 83.1 mph on average. Miller also generates a 35.6% whiff rate against them. And yet, one out of every five right-handed hitters he faces ends up reaching first base on a walk.

How can a reliever have enough stuff to dominate a plate appearance while giving the hitter so many chances not to face him? I think that's the reason the Boston Red Sox bet on him at the trade deadline. Miller still has the tools that made him attractive in San Francisco. His sinker averages 96.9 mph and carries enough movement to attack a hitter's hands. Against left-handed hitters, the results are even more striking: a 34.8% strikeout rate, a 34.3% whiff rate and an average exit velocity of just 80.5 mph. When they do put the ball in play, 60.5% of that contact becomes a groundball.

He just needs to find the same groove against righties. Earlier in his career, Miller relied much more heavily on his four-seam fastball (49.7% usage in 2024). In 2026, San Francisco transformed his arsenal: The four-seamer fell to a 9.0% usage rate, while the sinker climbed to 45.0% and the slider to 40.7%.

The change makes sense. The sinker allows Miller to attack with velocity and downward movement, while the 86-mph slider gives him another look capable of generating swings and misses. In fact, it has produced an impressive 53.3% whiff rate.

Left-handed hitters are obviously the ones directly punished by this pitch mix. A hard fastball that moves in and a sweeping breaker that runs away is a tough combination to deal with, and makes guessing at the plate the only real path toward success.

Against a right-handed hitter, that equation changes. The slider no longer moves away from the hitter. It moves toward the plate, while the sinker retains its downward movement and runs toward the hitter's side. Miller still has two pitches capable of producing the result he wants. The key is getting the hitter to choose between them.

That is where the problem emerges. Miller is getting the ball into the strike zone too infrequently against right-handed hitters. His Zone% falls to 36.9%. That allows the hitter to change the objective of the at-bat: Instead of deciding which pitch to attack, he can wait for Miller to find the strike he considers perfect enough to swing at.

The numbers suggest the problem isn't that right-handed hitters have figured out how to hit his pitches. Miller still generates whiffs and keeps exit velocity down when they make contact. The problem is that he isn't always forcing them to make a decision. And that can change an entire plate appearance. A pitcher with a 96.9-mph sinker and a 115 Stuff+ can live with some contact in the zone if that contact looks like the kind Miller is already producing. A weak groundball can be a way of winning the at-bat.

The problem comes when the search for the perfect pitch turns an advantage in stuff into an inability to throw strikes. That is where the Red Sox can make a difference. Andrew Bailey doesn't have to rebuild Miller's arsenal. But he does have to get the left-hander comfortable trusting what is already there. If the sinker can produce weak contact in the zone, the strike doesn't have to be perfect to be useful. And that could change the at-bat from the first pitch.

That also helps explain why Miller's profile makes sense in Boston. His sinker has already produced a 53.3% groundball rate, and a reliever who can keep the ball on the ground can have a useful weapon at Fenway Park. But that advantage only exists if Miller allows his pitches to do their work in the strike zone.

That is what makes this gamble so interesting. The tools are already there; the question is whether the Red Sox can turn them into a more complete weapon against hitters from both sides of the plate. If they do, the payoff will be huge, perhaps altering the outlook of the bullpen both now and in the future.