It was the end of an era for one of the greatest teams in Italian football history.
Herrera had built a formidable team at Inter Milan
Inter Milan went into their most famous clash with Celtic after back-to-back European and Intercontinental Cup successes in 1964 and 1965.
Since that fateful day in Lisbon, however, the Milanese giants have failed to win either competition again.
Defeat by the Glasgow side is seen in Italy as the closing chapter in the history of the Grande Inter (Great Inter).
Indeed, it was part of a disastrous spell for the Nerazzurri as they missed out on the domestic league title, lost a Coppa Italia semi-final and fell to Celtic in the space of a few days.
It was a collapse which few people had seen coming.
"Inter were considered to be the strongest team around," explained Alberto Cerruti, of Milan-based sports newspaper La Gazzetta Dello Sport.
"They were thought to be almost unbeatable and nobody really thought they could lose to Celtic - especially after going a goal ahead."
The build-up to the game had been characterised by talk of the "Tripletta" - a treble of European Cups for Helenio Herrera's side.
Club president, the oil tycoon Angelo Moratti, had helped to assemble a team reckoned to be one of the strongest in the world - a feat which has constantly eluded his son Massimo who currently runs the illustrious club.
Italian internationals like Giacinto Facchetti, Alessandro Mazzola and Mario Corso formed the backbone of the side - all three made more than 400 league appearances for Inter.
Around them the team relied on the brains of Spanish midfielder Luis Suarez and a tough, uncompromising defensive line.
The result in Portugal was a blow from which they never really recovered despite winning another league in 1971 and losing the European Cup final again in 1972.
"You could see that the team had given its all but was starting to fade after three years at the very highest level," said Cerruti.
The result sparked great celebrations among fans of Inter's city rivals Milan, but it also opened the eyes of Serie A supporters to the quality of the Scottish side.
"Jimmy Johnstone was considered to be Celtic's most dangerous player and a symbol of that side - but Tommy Gemmell was also highly regarded," confirmed Cerruti.
"Celtic also gave AC Milan a fright in the same competition in 1969 when they came to the San Siro and drew 0-0 in the quarter-finals.
"And then they nearly won the cup again in Milan when they lost out to Feyenoord after extra time."
It means that Celtic still enjoy a strong reputation in Italy despite the four-decade gap since their last European triumph.
Certainly, if you mention their name to any Inter fan old enough to remember events in Lisbon, it still causes them to wince with pain.
BBC One Scotland will screen Jock Stein: You're Immortal on Friday 25 May at 2305 BST