If the Sox do have concerns about their rotation, then the only option at this point - since trading any of Sale, Price and Eovaldi is not likely at all - should be to really focus on the bullpen and shorten games.
Chapman is certainly unpopular among Sox fans, but if he opts out, he does become the premier reliever on the market. I do suspect he opts out, since he can make more in a deal now that he will with his option years in NY.
If the Sox don't pay for Chapman, always possible on a resetting team, the next tier of relievers on the market on not so attractive. Hector Rondon is certainly very underrated and under appreciated, but he is also not exactly among the game's best. Arodys Vizcaino is a terrific reliever, but he is rarely actually healthy enough to pitch. Betances is currently injured, but might be available, but also carries The Former Yankee Stigma aka Ramiro Mendoza Syndrome.
I suppose Kenley Jansen might opt out...