I'm sure management knows more about their injuries and the future possibilities of health and meaningful innings. If they view Sale and Price as probably pitching 300-350 IP combined, next year, then I'm sure you'd call it hope not science. If they are relatively certain of 350 IP, then their plan will reflect that fact. If their relatively certain of 200-350, they'd probably plan on getting starter depth, if they are going for a ring.
It's interesting to note that the IP leader from 2018-2019 is Justin Verlander- the old man.
One could argue, the Sox limited Sale's IP last year, even though he might have been able to pitch more, if needed, but let's go by just IP'd. 2018-2019 IP in the AL- Yanks vs Sox:
5. Porcello 354 (Likely not on the Sox, next year)
17. Tanaka 328
21. ERod 315
25. Sale 305 (More than Paxton)
26. Paxton 304 (Who next year, astonishingly, will be one year younger!)
28. Price 283 (15 teams in AL, so top 20 is in 2nd tier out of 5.)
35. CC 257 (Retiring)
42. German 226 (some as RP'er)
51. Severino (The iron man)
67. Eovaldi 164
85. Green 140
88. Velazquez 137
92. Johnson 134