In production, these files should already exist.
I was trying to explain to a non-native English speaker that this sentence needed the comma after
production, but when trying to explain the grammar as to why, I realized it was only a feeling I had. When I looked it up, I found this link, which says if a prepositional phrase is less than four words at the beginning, then it does not need a comma.
So, just to clarify, does this need a comma? And what is the reasoning behind the answer?