General:
* Forgotten
* Unlearned
* Unremembered
* Unknown
* Unfamiliar
More specific:
* Improvised (if talking about something like a speech or performance)
* Spontaneous (if talking about a reaction or action)
* Unrehearsed (if talking about something like a performance)
* Unscripted (if talking about something like a dialogue)
The best antonym will depend on the specific context in which "memorized" is used.