Still a lot of season left so this may work itself out over the course of the next several months but, Sox will have to decide between Rodon and Lynn this offseason.
Barring an injury or a completely unforseen trade, you can pretty much lock these 4 guys into the 2022 rotation:
Giolito
Kopech
Cease
Keuchel
That leaves one spot for Rodon and Lynn, who have been by far our #1 and #2 starters this season.