Tyron Guerrero's Remodeled Sinker Has Given Red Sox's Bullpen A New Lethal Weapon

Guerrero last pitched this past Monday, and then the Red Sox announced that he had been placed on the 15-day injured list due to inflammation in his right shoulder. That night, he was able to record just one out against the Seattle Mariners in his 38th relief appearance of the season. He allowed three hits and an equal number of runs and walked a batter before wasting a two-run lead. Clearly, something was off in his game.

In 23 of his 38 relief appearances, he has secured at least one inning without allowing runs or inherited runners to score. In that span, he has accumulated 18 holds, a statistic that, although it does not define a reliever’s dominance by itself, offers another interesting point about how middle relievers get the necessary outs to keep the team with the lead. Every one of Guerrero’s accomplishments, no matter how small it may seem, has meant the biggest leap of his career.

I confess that, when the Red Sox began using him, my biggest faith was in the strength of his prodigious arm. So Boston deserves a lot of credit, because Guerrero has brought more than his supersonic velocity with a sinker that averages 100.2 mph, stability and strength to the bullpen. Along with the sinker, he has an uncomfortable 91.5 mph slider that has left the opposition with a .123 average.

In all his previous experience, he had thrown only four sinkers. It was in 2018, when he was pitching for the Marlins. So, the experiment worked, and Guerrero has gone through every inning trusting his sinker on 85.7% of his pitches.

No reliever who has pitched at least 30 innings this season uses the sinker or another pitch more than Guerrero—the only one who comes close is Kenley Jansen, who throws his cutter 84.6% of the time. As such, the big story here is what Guerrero is accomplishing with his primary lethal weapon.

Among several elite pitchers with elite pitches, the results show that Guerrero’s great dominance of the sinker has a place at the highest level. He has the highest whiff rate and the lowest xSLG allowed among the 24 relievers who have finished at least 100 plate appearances with a sinker. He also stands out with the second-highest strikeout rate, being surpassed by just 0.3% by the Mariners right-hander, Eduard Bazardo. He also has the second-lowest xBA allowed, behind Padres left-hander Wandy Peralta.

And, you know, he has achieved that phenomenal impact throwing the sinker 85.7% of the time, which seems crazy. Now let’s go a little deeper, because the breakdown by profiles takes us to a couple of interesting details. Let’s look at these two graphs.

On the left, we have Guerrero’s sinker whiff rates against right-handed hitters. Its power remains in the high-and-inside part of the zone, the same areas where left-handed hitters miss his pitches. There is not enough of a sample to compare the previous sinker that he threw, but this year it excels with -5.2 inches of induced vertical break compared with the MLB average. That is the main effect of his adjustments that is helping limit the impact of his opponents, especially against left-handed hitters, who have a 32.9 whiff% and only a .230 xSLG.

On the other hand, right-handed hitters have been more difficult to beat. His whiff rate drops to 10.8%, with a dangerous rise to a .377 xSLG and .339 BABIP. Guerrero still has control against both profiles, but he will need more from his slider and splitter to try to reduce contact.

It is possible (if not likely) that Guerrero will be back in a couple of weeks. It does not appear, at least for now, to be an injury that projects a prolonged absence. Boston’s expectation, for now, is that he could return around September 16 or shortly after. His arm has been key and, for the Red Sox, his presence in the late innings of games will be necessary down the stretch of the season.