Keuchel turns the average hitter he sees 3rd time through into peak Jose Abreu. The Sox have the deepest bullpen in baseball. There's no reason to push your luck with Keuchel in most situations he's out there.
If you don't overextend him, Keuchel is still a perfectly fine twice through the order/5 inning SP.
Ask him for anything beyond that and it gets very dicey.
Even if he turns things around, for me at least, there's nothing he can do to surpass Cease as the playoff #4. Maybe he can steal a bullpen role but he's not someone whose stuff is going to play up in relief.
There's no reason for him to even be a once through the order guy with Cease as the #4 and an ultra deep bullpen. For a lot of teams, though, yes I think Keuchel would still have a postseason role but the Sox aren't one of them.