Pep Guardiola's men were held to a 1-1 draw in the away game against Unai Emery's men, and Mascherano feels that the tie remains wide open despite having home-ground advantage in Wednesday's return clash.
"The semi-final is still wide open," Mascherano was quoted as saying on the club's official website.
"And with the teams level on aggregate, people are a lot more cautious than they were before the second leg against Real Madrid, when everybody was sure we had already qualified.
"We knew that Madrid would fight, would throw everything at us, and the same will happen with Valencia, who want to reach the final as much as we do.
"We are going to have to maintain the same attitude."
Wednesday's game at Camp Nou kicks off at 21.00CET.
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