A lefty reliever.

The team brought in Buddy Boshers and Anthony Gose to compete with Tony Sipp for the left-handed reliever on the team. As much as people say that they don’t really need a left-handed reliever, we saw them chasing Francisco Liriano in 2017 to fill that role. Sipp is in the final year of his deal, so he could potentially bounce back and perform like he did in 2014-15, but he has not pitched well the past two seasons.
Tony Watson is by far the best left-handed reliever on the market. Like other free agents, he is probably looking for a 3-4 year deal after a decent 2017. However, especially for the Astros, he is 32-years-old, which is not an age the team likes to invest in long-term. The Astros can do without Watson, so they are probably only looking for a 1-2 year deal. They could also only be willing to offer $8 million per year for a lefty reliever.
If he is desperate enough, he could accept a one-year deal to pitch with his friend Gerrit Cole and potentially go to the World Series. He has the advantage though. Most teams could use a left-handed reliever.