Interestingly, as u/artfulorpheus explains in this thread, Eastern Europeans may have been most familiar with the Buddha as a Christian saint! However educated Europeans would also have read the works of early Christian authors, which occasionally mentioned believers in the Buddha. Clement of Alexandria, while discussing Indian philosophers/gurus ("gymnosophists" as they were called in Greek) in his work Stromata, says that some of them worship "Boutta" as a god. The later Church Father Hieronymus (or St. Jerome in English) seems to know more of Buddhism, knowing of the legend of the Buddha's birth and calling "the founder of their belief-system" in his text Against Jovinianus. Someone with more knowledge of the mediaeval period could expand on if Buddhism is mentioned in any of their contemporary sources. I have read that Christian European travellers to Asia (William of Rubruck, Marco Polo) discussed Buddhists while calling them simply "Idolaters", but a medievalist could expand on that. I hope this is a decent answer at least!