The 21-year-old winger - a recent Socceroos call up - flourished on loan at Danish club Esbjerg from parent club FC Midtjylland last season.
And the move to Portugal, where big name opponents such as Benfica, FC Porto and Sporting Club await, presents a significant career opportunity for the former Adelaide United flier.
Built into the terms of the loan is an option for Pacos, who finished 13th in the Primeira Liga last season - to make the move permanent.
Playing alongside Mabil, should he nail downs first team spot, will be breakthrough Socceroos defender McGowan, who inked a two-year with the club in May.
Mabil made 25 Danish Superliga appearances for Esbjerg last season, bagging four goals and claiming a series of assists.
