The Belgians won all three of their group matches at the European Championship to set up a match against defending champion Portugal on Sunday in the round of 16.
Portugal will face Uruguay on July 30 for a place in the quarters after ending Iran's hopes of reaching the knockout phase for the first time.
Portugal star Cristiano Ronaldo's fourth-minute header clinched a 1-0 victory over Group B opponents Morocco in Moscow.
Santos is clear in his belief that Portugal will not be caught out by any surprise Spanish tactics following the last-minute upheaval.
Portugal opens against Spain on June 15 and then plays Morocco on June 20 in Moscow, where the team is based. The Group B schedule will conclude against Iran on June 25.
Ronaldo's presence had earlier elicited cheers from the 100 or so fans in attendance, with a similar number of media present along the touchline.
Portugal head to the World Cup as European champions but few would place them among the favourites in Russia.
Fernando Santos opted instead for the youth of Andre Silva and Goncalo Guedes to play alongside superstar Cristiano Ronaldo.