It's a bit hard to properly articulate what Randy Choate was for the Cardinals from 2013 to 2015.
In an era in which relief pitchers whose fastballs top out in "only" the mid-90s can be lost in the shuffle of Major League bullpens, Randy Choate did not top 89 miles per hour in his three years with the Cardinals.
To clarify, this isn't to say Choate didn't average 89 MPH. According to PitchFx, 89 was the maximum velocity of the pitches Choate threw in St. Louis. Choate did not fit in with what we perceive as the modern flamethrower bullpen: if Aroldis Chapman, Craig Kimbrel, or Trevor Rosenthal is the reliever equivalent to a supercomputer, Randy Choate is the equivalent of a perfectly adequate hammer.
Of course, a hammer sounds worse than a supercomputer, but it is considerably cheaper and for specific purposes, like if you need to hammer something, you'll be glad you have it. (I swear I don't own any stock in a hardware store. Yet.) And Randy Choate had one role with the Cardinals, and that was to record outs against left-handed batters. And even though he lacked overpowering stuff, for his first two seasons in St. Louis, Choate did just that.
Here are the best pitchers in 2013 and 2014 in baseball against left-handed batters by opponent OPS.
For two seasons, lefties facing Randy Choate had a lower OPS than Carlos Martinez had at the plate during the same time period. It wasn't necessarily glamorous (okay, it definitely wasn't glamorous) but it was effective.
But in 2015, things changed for Randy Choate. Now, he was not a complete disaster against lefties, but instead of handling them like they were so-so hitting pitchers, they managed an OPS of .695. And against righties, batters were OPSing .820, equaling the 2015 totals of Adam Lind and Mookie Betts.
For a reliever to hold opposing lefties to a .695 OPS is perfectly acceptable if he also serves another purpose. However, Choate, who turned 40 during the season, existed to mow down lefties alone, and so in spite of a renaissance at the plate, the Cardinals made no particular effort to retain Randy Choate for 2016.
Not that there were expectations for the Cardinals to re-sign Choate, particularly after he was passed over on the 2015 NLDS roster in favor of Tyler Lyons. But this does leave the Cardinals without a true LOOGY (a Lefty One-Out Guy, a left-handed reliever whose sole purpose is to face the opponent's most dangerous lefty bats). And while the Cardinals almost certainly will not apply a LOOGY as religiously as they applied Choate, it is only natural that certain players will take on the lion's (or Lyons (sorry)) share of tough lefties.
Although the Cardinals' 2016 bullpen is not yet set in stone, the prime candidates are the same seven names currently listed atop the team's depth chart. The following table lists their 2015 statistics against lefties, with the exceptions of Seung-hwan Oh, who has yet to pitch in Major League Baseball, and Jordan Walden, whose 2014 and 2015 stats I combined because of a very sparse 2015 pitching resume.
In this group of six pitchers (and in a group of seven pitchers which would include Oh), there are two lefties. However, neither Rosenthal nor Walden, who have easily the lowest opponent OPS against left-handed bats, are one of them. The two lefties, rather, are Tyler Lyons, a starter by trade who will need to begin the season in the majors to avoid exposure to waivers, and Kevin Siegrist, who has fared better against righties than lefties in his career.
As far as the LOOGY archetype, neither actual lefty seems to be a great fit. Lyons may not be used as a long reliever, per se (at least not in the Joe Kelly, "Ferrari sitting in the garage" mold), but the team would likely prefer to keep him at least more stretched out than a guy who faces one batter per night would be. And Siegrist, even if you attribute his "reverse split" to a statistical aberration rather than a reflection of his true talent, has been so effective in general that he projects more as a 7th or 8th inning guy than as a specialist.
As for Jordan Walden, there is a very simple explanation: he has been really good against everybody. In 2014, Walden held lefties to a .599 OPS, but he held righties to a .488 OPS.
Trevor Rosenthal, on the other hand, has fared better against lefties (.534 opponent OPS for his career) than righties (.675 OPS). But Rosenthal, who has easily the most defined role in the Cardinals bullpen, is not going to be abandoned as closer anytime soon, and for very good reason: while Mike Matheny will ideally conserve Rosenthal in 2016, keeping him out of some three-run leads in the name of long-term effectiveness, he is still the team's safest choice for high-leverage pitching situations, regardless of opponent handedness. In a sabermetrically optimal world, this would mean Rosenethal would get looks against lefties (or righties, for that matter) in high leverage situations before the 9th inning, but that's a battle for another day.
But this brings me back to Tyler Lyons. While being a true LOOGY is not likely (and probably not optimal) given his endurance, he was trusted enough as the de facto role holder to win it over Randy Choate for the NLDS in 2015. Of all of the small sample sizes used to evaluate pitchers based on efficacy against lefties, Lyons has the smallest sample of the group at 63 batters faced last season. And like most pitchers, Tyler Lyons was more effective coming out of the bullpen than as a starter.
There are many questions about Tyler Lyons in the bullpen that have yet to be answered. But on a roster with imperfect fits for the top choice to face tough left-handed hitters, Lyons appears to be the one which makes the most sense.