Let's reorder the sentence:
Fortunately, the necessary creative insight has already been supplied by Euclid, the Greek mathematician.
An article is needed when we put such a remark after a person's name, and we often keep that article when we move the clause in front of the name:
Fortunately, the necessary creative insight has already been supplied by the Greek mathematician Euclid.
Incidentally, we can use an indefinite article, too:
Fortunately, the necessary creative insight has already been supplied by Euclid, a Greek mathematician.
Using the instead of a, however, implies that most readers will already be familiar with Euclid. It's as if the author is emphasizing: "Yes, I mean that Euclid."
Both constructs are used in writing, however, whether the person in question is a household name or not. I found these four quotes using various Google book searches:
Buoyancy was first discovered by the Greek mathematician Archimedes.
This was first discovered by the German zoologist, Hans Spermann, just before the First World War.
The public attention has been drawn to the fact of the discovery of what is known as a "new star," first seen by a Scotch astronomer, Dr. Copeland, some time in the latter part of January.
This phenomenon, called osmosis, was first observed by a French physicist, Jean-Antoine Nollet, in 1748.
That said, an article isn't always required, as these three sentences show:
The word "robot" is used for the first time when Czech writer Karel Capek writes a stage play about robots trying to take over the world. (David Jefferis, 2006)
The definition of "robot" varies widely .. but the word is usually used to mean a machine possessing certain human attributes. It was first coined by a Czech writer, Karel Capek... (Proceedings, Royal Institution of Great Britain, v. 54, 1982)
This word was first coined in 1920 by the Czech writer Karel Capek. (R. Garg, 2011)
Bottom line: When it comes to articles, hard-and-fast rules do not always apply, as these three excerpts illustrate.