Messi, who grabbed the winner against Sevilla to score in his 15th straight La Liga match, believes the team must keep faith in their style to prevail.
"We must not drive ourselves crazy about the result in Milan or the first half against Sevilla," he told Barca TV. "We have to pull ourselves together because there are some decisive games coming up."
Although a 0-0 draw would be good enough for Barca thanks to the away goals rule, Messi added: "In front of our own fans we do not have any other option than to go for a win against Real Madrid.
"We cannot start thinking that a goalless draw would see us through. We have to be at our best against Real Madrid, because otherwise we will not get through the tie."