Djalo called up to replace Varela
Varela has been struck down by an injury to his left ankle, according to his club Porto.
Djalo, 24, was born in Guinea-Bissau but has already represented Portugal at youth level.
On Monday, Besiktas winger Ricardo Quaresma was summoned to replace captain Cristiano Ronaldo, who injured his right ankle in Real Madrid's 0-0 draw at Real Mallorca on Sunday.
Portugal host Cyprus on Friday before visiting Norway on 7 September in the first two matches of their EURO 2012 qualifying campaign.