It's no secret that the Mariners struggle against left-handed pitching. Their record against lefty starting pitchers sits at just 10-13, contrasted with a much nicer 25-18 against right-handed starters. The bats also go quieter against southpaws, slashing .244/.308/.416 against them, as opposed to .266/.340/.449 against right-handers. While Dae-Ho Lee and Franklin Gutierrez have pulled their weight in these spots, the rest of the lefty-heavy lineup hasn't produced as well.
A large portion of this problem appears to be lack of production from the leadoff spot. As of late, Scott Servais has been running out Ketel Marte as the table setter against lefties, but anyone running a 3.5% BB% like Marte is currently posting shouldn't be allowed anywhere near leadoff. Leonys Martin has been a nice surprise this season, and slots in well in that spot against right-handed starters, but his career-long struggles against lefties have continued this season, with his slash line sitting at .224/.278/.408 against southpaws - not the on-base production you want for a leadoff man. Norichika Aoki - king of the meh - has curiously struggled mightily to a .250 OBP and one (!) extra-base hit against same-handed pitching, despite running reverse splits throughout his career. And besides, considering how awful he's been on the basepaths this year, does anyone really want to see him there, anyway?
So the three "traditional" leadoff men on the Mariners roster all have some big flaw against southpaws. Ideally, all three of these players would comprise the bottom third of the order when the team is facing a left-handed starter, and the order is largely irrelevant.
But this creates a new dilemma - who do you put in that spot? In a perfect world, you would like your leadoff man to be adept at running and getting on base, but this is currently a luxury the Mariners do not have. They do, however, have a right-handed bat that has been destroying lefties, walks a ton, doesn't strike out too often, and has a little bit of pop in his bat. Who is this man, you may ask?
Yep. Chris Iannetta should be the Mariners' regular leadoff hitter against lefties from here on out.
Now, I know this sounds silly at first glance, and might bring back repressed memories of the failed Logan Morrison: Leadoff Hitter experiment from last season. Iannetta has never even batted in the leadoff spot in his entire career! But it isn't as ridiculous as you might think - there are several reasons this idea is nothing like leading off with LoMo, and it may have a shot at actually working.
The first point is the most obvious one - Iannetta draws a ridiculous amount of walks. While his BB% of 12.6% on the year is a little down from his career rate of 14%, that's still an excellent number, good for second on the team just behind Seth "Dad" Smith. It's also well above what Morrison put up while he was here - 6.6% and 9.2% in 2014 and 2015, respectively. You want your leadoff hitter to get on base consistently, correct? Well, Iannetta has done just that this season, as his OBP is currently sitting at .348 - putting him one percentage point behind Jason Castro for tops among American League catchers with at least 150 plate appearances. His strikeout rate is also the lowest it's been since 2011, currently sitting at 21.2%. A K rate like that might be a little high for a leadoff hitter, but the walks do more than enough to help offset them.
The second point is that Iannetta has absolutely clobbered left-handed pitching all season. His wRC+ against lefties currently sits at 164, and 17.1% of the time he faces one, he works a walk. The BABIP of .391 is almost definitely going to come down a fair amount over the course of the season, but for his career, Iannetta has posted a 134 wRC+ against lefties, with a BABIP of almost one hundred points lower than what he's running this year. His ISO against lefties this year is also a little down from his career mark - .161 in 2016 against .220 for his career. Even with a significant drop in BABIP, Iannetta can still be a real weapon against lefties with the walks alone. And if that power bounces back a little bit to his career marks? Watch out.
The third point is that Iannetta sees a fair amount of pitches, currently sitting at 3.82 P/PA. While this isn't a spectacular mark - Franklin Gutierrez of all people actually leads the team with 4.37 - it's better than both Marte and Aoki, who are sitting at 3.80 and 3.69 P/PA, respectively. Some of this might have to do with the dip in strikeouts for Iannetta - after all, because you need to see at least three pitches to strike out, that number is going to be higher for the high-K guys. But if he can keep working walks, or even up them by a tiny bit, his P/PA should climb, as well.
"But!" you might be saying. "Chris Iannetta has been really streaky all year! What happens when he goes cold again? And he's slow even by catcher standards!"
The streakiness is definitely a concern. Iannetta posted a woeful 53 wRC+ in May this season despite the fun walk-off homer against the Rays. A big factor in this rough month was an absolutely brutal 2-for-28 stretch to close out May, with only two walks to boot. He did have a solid April, though, posting a 106 wRC+ with a 13.8% BB%. The alternation of good and bad months for Iannetta is actually a trend that dates back to last year, when he posted a -1 (!!!) wRC+ in April, 62 in June, and 11 (!!) in August. In the other months, however, he posted wRC+s of 129, 161, and 127.
This shouldn't be too much cause for alarm, though. For one, with the exception of a below-average 6.1% BB% in August 2015, the walks have remained a constant for Iannetta through thick and thin. Even this past May, when a lot of people were growing worried that catcher was going to be a black hole again, Iannetta was still walking at a rate of 10.7%. Once the walks start to drop significantly, that's the real time to be concerned about him.
No one will argue that Iannetta is anything close to fast, or even average speed on the basepaths. However, speed isn't always necessary for a leadoff hitter to be productive - after all, John Jaso (miss you) spent a significant chunk of time while with the Rays at leadoff and was very productive there despite having only fifteen career stolen bases. It is much more worthwhile to be a smart baserunner than a fast one, and Iannetta has done nothing so far to show me that he doesn't know what to do out there.
Chris Iannetta: Leadoff Hitter might not work out. It might not even be tried at all. He still absolutely belongs in the bottom third of the order against righties - his 73 wRC+ against RHP this year proves that. But the triumvirate of Marte, Martin, and Aoki at leadoff hasn't worked against southpaws. Chris Iannetta at leadoff against them has a chance to.