It could also not matter.
Houston has been struggling with the closer role for the past 2 seasons and it hasn't hurt them.
If the bullpen has good pitchers, the closer becomes a little less relevant. Most people do seem to think that relief pitchers are inconsistent from year to year. This is backwards. The truth is, pitchers who are inconsistent from year to year become relief pitchers. There are plenty of relievers who consistent performers who can be had to build a bullpen. And the Sox will still have Thornburg, who was a very good reliever anf closer before the Sox acquired him. His struggles this year are very likely related to an injury and excessive time off. As long as there are no lingering effects to his shoulder, he is an excellent bounce back candidate to start the new 2019 bullpen, along with Barnes, Brasier, Poyner, and Hembree and possibly even Velasquez. Carson Smith is a non-tender candidate whose return is and should be in question.
So if the Sox go the free agent route, for example, there are a few intriguing candidates who won't break the bank like Kimbrel, including Jeurys Familia, Brad Brach, and Adam Ottavino. It's also possible to convert a talented starter to a reliever/closer role. (Obviously a closer role would be closer to starter money, making the transition more appealing.) Two candidates that leap out at me right away who cannot handle a full season as a starter but are very talented pitchers are Garrett Richards and Matt Harvey. (Tyson Ross should also consider the idea.)