The Portuguese side lead 1-0 after the first leg, but Pellegrini insisted there was still another match to play and hopes his defence can be strong as they look to reach the last eight.
"It’s a 180-minute match, 90 minutes of which we’ve already played,” he told reporters at a press conference.
"We’re currently at a disadvantage and have got another 90 minutes to turn the match around, taking into account the additional value of away goals.
The former Real Madrid trainer is not predicting any surprises from the visitors on Wednesday, and hopes his side can avoid making any crucial mistakes.
"I don’t think Porto will change their style of play. They always have certain characteristics on the pitch, and I’m sure they won’t be concentrating on defence, but will be determined to score to increase their advantage," he continued.
"The most important thing we can do is maintain our confidence and determination to win.
"It’s important that we keep our nerves and anxiety under control, as if we’re too nervous we’ll make mistakes."
The 59-year-old also feels an early goal could calm the side down and give them a great chance, though he admitted it was not essential.
He added: "An early goal would be ideal, but we’ve got 90 minutes to score. We’re focusing on playing a clever match and ensuring we have a positive result at the end of the 90 minutes."