I think you and other have over-simplified things. You don't go out and get starters like you're buying groceries at the supermarket. If Kluber and a bunch of other guys have taught us nothing else, it's that the starter market is tricky and, in my opinion, vastly over-priced.
I used to listen to a radio show called "Wait, wait, don't tell me" and one Sunday they talked about Whole Foods and how they were joining a coop so that could save up to buy a pear. To me that's the starting pitcher market in MLB.
Plus everyone seems to have forgotten that on 1 August Sale was about to return (11 August?), as were Houck and Whitlock. All three were known quantities and paid for.
Since roughly August 11, the Sox have pretty much stopped going with reliever starts/games. Even with Paxton out, the Sox rotation right now is Bello, Crawford, Sale, Pivetta, Houck, Pivetta, and Sale--and their WAR's are 4.0, 2.2, 1.6, 1.4, and 1.3.
Also, as I have argued far too often, the Sox are about to complete 48 games with just 3 days off--a killer for any pitching staff. Those 48 games end tomorrow, and it's worth noting that in the most recent 8 games the Sox pitching staff has given up 3, 4, 0, 8, 3, 4 (13 innings), 3, and 2 runs.