United cruised to a comfortable 4-1 win to reach the final 7-1 on aggregate. But Valverde insists the scoreline didn't tell the true story of Thursday night's game.
Valverde said afterwards, "We played very well and we were in the match for 80 minutes, playing very close to their box, pushing them, but then we played very badly the last ten minutes. Football is 90 minutes, not 80 or 75 minutes.
"The result is overwhelming considering we had them in their area and we were pushing to get into the tie after doing the most difficult thing, making it 1-0. But the last ten minutes were very bad, with a moment of weakness.
"The tie wasn't a walk in the park for United in either the first or second leg; it was much more even than the scoreline suggests. We stood up for it, but we have to stay on our feet until the end, because these teams don't forgive you."