Athletic were hammered 5-0 in Jeddah, with Barca leading 4-0 at halftime.
Valverde said afterwards: "It was a match where we started well, but as soon as Barça scored, we felt the blow too much. If you hang your head against teams like these, they'll run all over you; that's what they have to do.
"The 4-0 scoreline at halftime was painful, and the second half was torture. We had to try to prevent it from getting worse because we didn't have enough spirit. A bad match, one of the worst we've played in a long time, and on the day we were playing for the Super Cup here. We have to pick ourselves up, and that's what we're going to do."
Valverde also said, "What happened today isn't something that happens regularly. We're a team that always puts up a fight and tries to give our all in every match... but today, I don't know, it's something that doesn't usually happen to us.
"Maybe we thought that sitting back would help us do better, but it didn't... but we have to face the situation.
"Now we have to pick ourselves up after a setback like this, which has been awful for us. You can lose in many ways, but when you're losing 4-0 at halftime, it means the opponent has been far superior to you."