After Jerry Blevins broke his forearm on a comebacker in mid-April, the Mets had a season-long lefty-specialist hole in their bullpen. While Rule 5 pick Sean Gilmartin had a solid season and rookie Dario Alvarez showed flashes—most notably striking out Bryce Harper in a pivotal division matchup—left-handed relief depth remains a problem for the Mets entering the 2016 season. This hole was obscured to a degree in 2015 by the frequent use of Jeurys Familia and the heavy workload placed on the starting rotation, but the Mets would benefit from having a better bullpen option for tough, left-handed batters.
Enter Antonio Bastardo. The 30-year-old lefty is coming off his finest season, registering a 2.98 ERA and 3.33 FIP with his typical high-strikeout, high-walk peripherals. Most notably for the Mets, Bastardo regained his effectiveness against lefties. After allowing wOBAs of .271 and .291 to them in 2013 and 2014, respectively, Bastardo posted a career best .211 wOBA against same-handed hitters in 2015.
Unfortunately, caveats abound with Bastardo's 2015 performance. While his walk rates to lefites decreased, most of the improvement likely comes from the .189 BABIP he induced. That mark is more than 50 points below his career BABIP mark and almost 70 points below his 2015 xBABIP—expected batting average on balls in play—numbers.
Analyzing Bastardo's actual approach shows greatly-increased use of his fastball, which could have contributed to the decreased walk rate. However, the pitch effectiveness data does not indicate that the change in pitch mix had any significant effect beyond the aforementioned BABIP luck.
Even with these concerns, Bastardo still makes sense for the Mets. Hand waving a significant improvement away as BABIP is overly dismissive, as it's extremely possible there are other factors we're missing that make Bastardo's return to lefty-dominance sustainable. More than that, the rest of the left-handed relief market is quite barren; nearly every option is either old (Matt Thornton. Chris Capuano, Joe Beimel), dealing with injuries (Blevins, Sean Marshall), or simply not very effective (Eric O'Flaherty, Joe Thatcher, many others). Bastardo is still in his peak years, throws hard for a lefty, and offers the best risk/reward profile of the options available to the Mets.
Bastardo is hardly a slam dunk solution to the Mets' left-handed hole, but the 2015 free agent market doesn't offer any single player who is. Therefore, the best option is to bring in a couple different arms and let entropy do its work. Given Bastardo's presumably small acquisition cost, he'd make a solid addition to the Mets' left-handed relief options—joining Gilmartin, Alvarez, hopefully a re-signed Blevins—on a short, low-salary major league deal.