"Reasonably" is an adverb, so it must describe "how" a verb is being performed. E.g. "He acted reasonably". He acted, yes, but the way he acted was "reasonably".
"A reasonable-enough assumption" (I've hyphenated "reasonable-enough" to illustrate that it's a compound adjective, although it's rarely if ever written that way, sadly) is another way of saying "an assumption that is sufficiently reasonable." "Reasonable" is an adjective, so it modifies (gives more information about, or specializes) a noun.