The two nations clash in the first leg on Friday, with Ibrahimovic claiming that Cristiano Ronaldo's presence makes the Portuguese the favourites to book their spot in Brazil.
"Portugal have done very well so far and have a good team," the Paris Saint-Germain striker told Gazzetta dello Sport.
"They have some great individuals – with Cristiano, of course, they are favourites. It's going to be difficult for us. Qualifying for the World Cup is going to be tough."
Ibrahimovic has scored 46 goals in 94 games for Sweden.