New signing Tiago has been cleared to play for Chelsea by the Portuguese Football Association.
Tiago can play for Chelsea if he recovers from injury
The midfielder was called up to Portugal's squad for the Olympics but was ruled out of the tournament with a groin problem and returned to Chelsea.
Fifa has ruled that Tiago can make his Premiership debut, even though Portugal are still in the Olympic tournament.
Chelsea paid Benfica £8m for Tiago, who was in Portugal's Euro 2004 squad but did not make an appearance.
Portugal's Olympic squad also includes Manchester United winger Cristiano Ronaldo and Fulham's Luis Boa Morte.
Last month, the Portuguese Football Federation turned down a request by Chelsea and Manchester United to spare the 23-year-old Tiago and Ronaldo from the forthcoming Athens Games.