Los Leones sprung the surprise of the last 16 last week when they came away from Old Trafford with a 3-2 win courtesy of goals from Fernando Llorente, Oscar De Marcos and Iker Muniain.
However, a Wayne Rooney double, including a penalty late in stoppage-time, has given the Red Devils hope of progression ahead of the meeting at San Mames.
Confidence is expected to be high in the Bilbao camp, but Iraizoz admits they are not underestimating the Premier League champions and confessed their win in Manchester will count for little if they fail to reach the quarter-finals of the competition.
"This tie has not been won," he told Sky Sports
"Any 90 minutes in the San Mames is hard because the rivals change their system and we need to be focused from the first minute as an error could decide this.
"The atmosphere will be unique and it will be an incredible night for us to remember.
"I expect United to be more offensive as they need goals, that could be both positive and negative for us but we need to control the ball.
"The win at Old Trafford was historic, but we need to close the tie out and continue in Europe and then the enjoyment will be complete.
"But we must play without even knowing the 3-2 result from England, the coach insisting on us being mentally prepared."
Bilbao coach Marcelo Bielsa will be hoping De Marcos has recovered from a virus in time for the fixture, while Llorente is expected to lead the front-line despite reports claiming the Spanish international has missed the past week of training with a groin complaint.
