Zach Thornton is back, and the Mets hope the prospect returns looking more like the pitcher they saw in his second outing than his first. Thornton will be used in Sunday’s series finale, either as a starter or a bulk reliever, though the exact way in which he will be used has yet to be determined, interim manager Andy Green said. “He will figure into our mix at some point tomorrow,” Green said before the Mets hosted the Red Sox on Saturday afternoon at Citi Field. The 24-year-old Thornton has made two appearances for the Mets this season. In his debut, he lasted only 4 ¹/₃ innings against the Nationals, allowing four earned runs. His second outing was much better, a six-inning, one-run showing against the Phillies. It is uncertain if Thornton will have an opportunity to remain with the tea