Djalo replaces Varela in Portugal squad
Varela was suffering from pains around his left ankle joint, it said.
It was the second replacement in as many days after the federation said winger Ricardo Quaresma would replace Cristiano Ronaldo, who bruised his right ankle during Real Madrid's La Liga opener on Sunday and will be out of action for around three weeks.
The 24-year-old Djalo, born in Guinea Bissau, plays as a striker or winger. He was a regular in the under-23 side and was called into the senior squad for two World Cup qualifiers in 2008 but did not come off the bench.
Last Thursday, Djalo scored the decisive goal in the 90th minute of Sporting's play-off match against Danish side Brondby, allowing them to qualify for the Europa League's group stage.