PORTO, Portugal — There is an office at the end of a quiet corridor on the ground floor of the sports faculty at the University of Porto. It is not the sort of place that attracts much foot traffic, and it is not decorated as if it is intended to do so.

The room’s occupant, Vítor Frade, is retired from the teaching post he held at the university for more than three decades. He keeps the office, though, as a convenient place to receive the steady stream of visitors who come from across the world to pick his brain, seek his advice or simply hear him talk.

Over the course of his long career, Frade achieved no small academic success, but he could not be described as famous, not in the sense that soccer usually means it. Fans do not sing his name in stadiums, nor do they ask him for autographs in the street.

He was not a player of any great note. He has never managed a club. Instead, Frade, 73, is that rarest of things: one of soccer’s most noteworthy theorists.