The first leg at Old Trafford ended in a 2-2 draw and United will have to get a goal in the return leg in Portugal.
“The atmosphere will be enormous in the Estadio do Dragao so the team cannot fail,” explained Gonzalez to News of the World.
“It would be a moral tragedy and a huge blow not to finish off United after the exhibition we put on at Old Trafford but we can do it.
“It is not easy to silence United’s fans but we did that from the beginning until the end. In fact, I think the tie was unfair as we deserved to win.
“If we knock out the holders, we can even aim to win the title.”