Except he didn't say "are good," in which case it would have been appropriate to use
good as a predicate adjective modifying "doctors." He said "feel good," in which case he was using
good (incorrectly) as an adverb modifying "feel." In English, the adverbial form of
good is
well, in contrast to German, which uses
gut both as an adverb and an adjective. 😛