When the Pittsburgh Pirates acquired left-handed reliever Mason Montgomery from the Tampa Bay Rays, they were hoping they could develop him into a quality reliever. Despite his 5.67 ERA last year, he had solid underlying numbers (3.92 FIP, 3.56 xFIP, 3.55 SIERA), struck out 30.1% of opponents, and had a 123 Stuff+.
The key to unlocking Montgomery's full potential was harnessing his stuff, as he walked 12.9% of opponents, and throwing at least a third pitch, as he only threw a four-seam fastball and slider in 2025. However, the Pirates have not only gotten Montgomery to control his stuff better, but his newest pitch, his sinker, which he added in July, has helped him blow all expectations out of the water.
Montgomery threw the first sinker of his MLB career on June 23. Prior to that, he had some quality numbers. Despite his 4.71 ERA, Montgomery owned a 3.45 FIP and struck out 29.1% of opponents. However, he was still handing out a free pass at a rate of 11.8%.
The left-hander added the pitch to his arsenal when pitching coach Bill Murphy came up with the idea. "We thought that the pitch would just be something that he could throw naturally, and that's exactly what it has done. I think it's just freed him up," Murphy stated in an article to Adien Stepansky in August. Montgomery also stated he "Mentally, I feel good about throwing it" in the same article.
It's clear when Montgomery got fully accustomed to throwing his new pitch. Since the start of July, Montgomery owns a 0.62 ERA, 1.00 FIP, and a 0.62 WHIP in his last 29 innings pitched. He has struck out nearly half of the batters he has faced with a 45.8% K%. Along with that, he has cut his BB% down to just 6.5%. Opponents struggle to make any quality contact against him, with a 2% barrel rate.
Montgomery has simply been one of, if not the best pitcher in the sport since July began. His 39.3% K-BB%, .111 batting average against, 1.06 FIP, 1.22 xFIP, and 1.31 SIERA are the best among all pitchers with at least 20 IP since July 1. He also has the second lowest WHIP. Only six other relievers have an ERA under 1.00 in that time as well.
Despite adding his sinker halfway through the season, it has already become one of the most effective pitches of its kind. He has held opposing hitters to just a .198 wOBA with his sinker, which is the third-lowest mark of any pitcher who has thrown at least 150 sinkers. He has a 33.7% whiff rate on his sinker as well. The only other pitchers with a 30%+ whiff rate on their sinkers are Aroldis Chapman and Josh Hader.
The sinker isn't the only pitch Montgomery added. He also started throwing a curveball this year. However, it's clear that his sinker has become an ultra-dominant pitch and is a main reason for his breakout. He looks like a long-term closer for the Pirates right now.
