I've read a number of posts that state that Goff has struggled, or not been the same, etc., because Kupp was out. Kupp was special, no doubt, but he's one wrench in a box of wrenches.
Goff's got two great WRs in Woods and Cooks, a more than serviceable replacement in Reynolds, two steadily improving TEs, and a back that can catch a thief. So many tasty options!
If Goff can't make this type of adjustment then there's something wrong. Instead, as you pointed out, he killed it in the KC game. Goff has proven he's an all star, I think that includes being able to adjust to difficulties and changes. Surely he faced similar problems at Cal.