This is a fancy way of saying that Portuguese, unlike English, really doesn't like "tb" occurring together with no vowel in between, so people added one! (In other words that have a longer history you sometimes don't have the vowel written, but it's commonly still spoken, like advogado /a.d͡ʒi.voˈɡa.du/ 'lawyer', from Latin advocatus. IPA again from Wiktionary.)
The "e" in futebol -- written and spoken -- is an epenthetic vowel due to Brazilian Portuguese phototactics.
https://en.wikipedia.org/wiki/Epenthesis#Breaking_consonant_...
https://en.wikipedia.org/wiki/Phonotactics
This is a fancy way of saying that Portuguese, unlike English, really doesn't like "tb" occurring together with no vowel in between, so people added one! (In other words that have a longer history you sometimes don't have the vowel written, but it's commonly still spoken, like advogado /a.d͡ʒi.voˈɡa.du/ 'lawyer', from Latin advocatus. IPA again from Wiktionary.)