So, what’s causing this problem? Well, there are three major factors. First, the Mariners are extremely left-handed. Julio and Randy are the only right-handed everyday players they have. Naylor is left-handed, JP is left-handed, Cole Young is left-handed, and Brendan Donovan is left-handed. His replacement at third in Colt Emerson is also left-handed. Both of their best options at RF and DH in Canzone and Raley are left-handed. In general, left-handed hitters struggle against same-sided pitching across the MLB, and the Mariners are no exception. Luke Raley has repeatedly proven that he can’t hit southpaws to save his life, while the others have struggled more than usual. JP Crawford has had reverse or even splits in the past, but not so far this season. To put it simply, the Mariners don’t have enough everyday players hitting right-handed.
Second, the Mariners’ fix for this issue has failed. To get more right-handed bats against lefties, they planned to use platoons at RF, DH, and infield. The right-handed half of those platoons were intended to be a mix of Rob Refsnyder, Mitch Garver, Victor Robles, and Leo Rivas, but three of the four have been dealing with injuries this season. Victor Robles missed nearly two months with a pectoral strain, Mitch Garver has missed a bit of time with lower back tightness, and Rob Refsnyder was been playing through an unspecified knee issue. Rivas stayed healthy, but was struggling so much he got sent down to Tacoma a few weeks ago. The Mariners have tried to patch things up with internal candidates, calling up right-handers Connor Joe, Patrick Wisdom, and Jhonny Pereda to fill in as guys hit the injured list. Unfortunately, the band-aids didn’t do anything to stop the bleeding. Only Pereda has performed against left-handed pitching. As for the others, Connor Joe already dug himself a hole down to Tacoma, and Wisdom has been so bad he might not even get an offer from Korea this offseason.
Third, this LHP weakness also impacts the lineup against right-handed pitching. Canzone and Raley have been some of the Mariners best hitters this season. But they don’t trust them to hit against lefties, usually opting to pinch-hit for them when a lefty reliever comes into the game. So, the opposing teams bring in lefty relievers as soon as their starter is done, get Canzone and Raley out of the lineup, and then face a nerfed Mariners lineup for the rest of the game. Now, it would probably be better to just leave them in the game, but either way, it still hurts them offensively.
Their lefty/righty split is so bad that it drops their overall wRC+ from 119 to 109, the largest gap in the league. Despite this, the Mariners are still top-5 in wRC+ overall on the season, and over the past 10 games, the Mariners have been a bit better against left-handed pitching. Robles and Garver have been fulfilling their roles, Refsnyder has been a bit better, and the rest of the lineup has mostly improved their at-bats. Success in small sample sizes can leave as fast as it comes, but this is still an encouraging sign. In the regular season, they will face enough righties that it will mostly offset their struggles against lefties. But if they want to make it to the World Series, they will have to fix this weakness. Some of their biggest competitors in the AL have elite lefty starters, and it will absolutely cost them a playoff series if they run into the wrong team.