The match has been billed as the biggest in Malaga's century-old history as they bid to overturn a 1-0 deficit and book their place in the quarter-finals for the first time.
The southern Spanish city has been gripped by a debut Champions League campaign that had seen their side go unbeaten - and beat AC Milan to win their group - until defeat in Porto last month.
The importance of the game has been heightened further following UEFA's decision to ban them from European competition - should they qualify in the next four years - due to unpaid bills.
Malaga, fourth in La Liga, have been left aggrieved by the decision, claiming they had been made "scapegoats" and stirred emotion as they bid to prove themselves on the pitch.
"It's important that we keep our nerves and anxiety under control, as if we're too nervous we'll make mistakes," Pellegrini said at a press conference.
"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."
Porto midfielder Joao Moutinho scored the only goal of a first leg the Portuguese champions dominated.
Pellegrini does not expect the visitors to sit back on their lead with the influential Moutinho ready to return from a thigh injury that has kept him out since the first leg.
"I don't think Porto will change their style of play," he said.
"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.
"Our priority is not to concede any goals, as this would mean us having to score three times in order to win."
Brazilian forward Julio Baptista has recently returned after injury and admitted the significance of the match has not been lost on the players.
"This is an historic match for Malaga," he said.
"The fans will really enjoy the match and we'll be doing everything in our power to ensure we get the result we're all hoping for."