Formed at first by mutual admiration on the football pitch, it was then tempered by long, lively evenings in the bars and restaurants of European cities.
"I was devastated," says Willie Henderson, the former Rangers and Scotland forward, after hearing of his close friend Eusebio's passing.
"I spoke to him a few months ago and he was doing okay. He had a scare at the European Championships, but we thought things were progressing in the right direction."
The pair first crossed paths in 1964. Helmut Schoen was then manager of West Germany, but was also about to take charge of a Rest of Europe side to play Scandinavia as the region celebrated a centenary of football.
A month earlier, in April, Schoen had taken in Scotland's 1-0 victory over England at Hampden and had been so affected that he named four Scots in his squad: Jim Baxter, Denis Law, Alex Hamilton and a pacy, pint-sized, 20-year-old winger.
"I was at Copenhagen," recalls Henderson. "Eusebio was there, we met, got friendly and then confirmed the friendship with a night out in Denmark after the game.
"He didn't speak much English and I didn't speak Portuguese. Communication was limited. At that early stage, there wasn't too much said. But we were happy in each other's company."
In that halcyon period of glamour friendlies and thrown-together money-spinning XIs, they were to meet regularly. Mario Coluna, the Benfica captain who marshalled their midfield throughout the '60s, moved on to Olympique Lyonnais, but was first awarded a testimonial match in December 1970, and faced a UEFA selection which featured the likes of Johan Cruyff, Dragan Dzajic, Geoff Hurst, Bobby Moore, Luis Suarez and Henderson.
"We went out in Lisbon after the game," the Scot says. "We went for a meal. There was always a dinner and a few drinks. One of the good things about [Eusebio] was although he was a fabulous player, he was a very nice man."
He was to also square off against the Portuguese in the more familiar colours of his club. Eusebio brought Benfica to Glasgow in September 1965 for a friendly match which was watched by nearly 40,000. Tommy McLean scored once, Jim Forrest twice, as Rangers won 3-2.
The Portuguese later admitted to an affinity for Henderson's half of the Old Firm, inspired mostly by his relationship with the winger. "I would think that would be a big part of it," agrees Henderson. "He had been involved in some stuff with that other great player - wee Jinky [Jimmy Johnstone] - in Lisbon and he was telling me that people think that because of that, he was a Celtic supporter. But he did say that, through his friendship with me, he was a Rangers supporter."
After that game at Ibrox - there is a merry pattern emerging here - Henderson, alongside his Rangers team-mates, gave Eusebio a whirlwind tour of some of Glasgow's more effervescent establishments. "Well, he didn't want to go to the pictures . . . so we had a night out some other way!" laughs Henderson.
As the years passed, Eusebio's English improved and the two began to be able to have richer conversations - usually over something freshly poured. They would exchange presents and strips, too, as well as phone calls, and were on the cusp of meeting up again.
Henderson has been working on a new television show, focused on the greats of the international game and their connections with Scotland. He was ready to fly out to Portugal with a film crew to interview his friend. It was destined never to be. "I've got to have Eusebio as one of the top-five players of all time," he says. "He's a huge loss, to his family and football."