Qatar-owned Malaga are making their debut in Europe's elite club competition this season but have a tough task to secure a place in the last eight after losing to the 2004 winners in last month's first leg in Portugal.
"We have to know how to deal with any nerves," Pellegrini told a news conference on Tuesday.
"An excess of anxiety leads you to make mistakes," added the Chilean.
"It will be very important to go out and try to score from the first minute but we have to remember we have 90 to do it.
"We will try to play an intelligent match in all areas, a balanced game so that when the 90 minutes are over we have achieved a positive result."
The match at the Rosaleda is only the second meeting between the two sides after Feb. 19's first leg and features two of the meanest defences in the competition.
Malaga conceded only five goals in six games on their way to winning Group C ahead of AC Milan, while Porto let in four to finish second in Group A behind Paris St Germain.
"I don't think Porto will change their way of playing which is very well defined," Pellegrini said.
"We are not expecting them to come and defend their lead but to come and try to win and to score goals.
"The important thing is that we go out there with the conviction that we can get through."
Pellegrini will have Brazilian forward Julio Baptista available after a brief period on the sidelines with a muscle strain, but Portuguese fullback Eliseu has not recovered from a knee problem and was left out of squad.
"I am fine," Baptista said. "I trained all week and if the coach thinks I should play I am in perfect shape for tomorrow," added the former Arsenal, AS Roma and Real Madrid player.