United overwhelmed the Spanish side 4-1 at home thanks to goals from Marcus Rashford, Antony, Bruno Fernandes and Wout Weghorst, four days after their humbling 7-0 loss to Liverpool in the Premier League.
"(Each game) is always serious and we will play our best team," Ten Hag told reporters on Wednesday.
"Definitely, we have to put up a team that can adapt during the game. We know our opponent can, because they have really good players, they (can choose) a lot of variations. So we have to be aware of that and be ready to go and battle with them."
Playing their best team means the United boss will not keep midfielders Casemiro and Fernandes on the bench, even though both are one card away from suspension.
"They will play, both," Ten Hag said. "Last week we had a good result, but we have to (play) again - and we need our best players."
United will be without winger Antony, who missed training on Wednesday morning due to illness and did not travel to Spain with the squad.
The Brazil international adds to United's absences, with Argentina youngster Alejandro Garnacho also missing due to an ankle injury.
"I can't say exactly at this moment (when Garnacho will be back). But it will take weeks, it's a serious injury," Ten Hag said.
"It's difficult to make a diagnosis this quickly. But I saw him limping and that's why I took him off.
"He will be back for the end of the season... This season we can win a lot so he has to be ready to get back as soon as possible."
Anthony Martial, Christian Eriksen and Donny van de Beek are also sidelined with injuries but Ten Hag dismissed the idea of prioritising the Premier League over the Europa League.
"I think we can manage. We have players that can recover good in between games," he said.
"We are in contact with medical and performance (teams) about what the players can handle. When we are going into the final stages of the season, we can bring our best team."
