United were thoroughly beaten 3-0 by the visitors.
Ten Hag said: "That's always the question. You have to start right and 100% focused, when you don't everyone is eager. It can't happen when the opponent is more eager than you are.
“We had good spells of football only moments where our positions aren't good enough. You can't make such mistakes at this level, opponents will take benefit from it. You can't switch off and that's what happened.
"We have to work on that if we want to be successful. If you want to reach the targets you've set you have to be consistent."
On lifting players ahead of the game against Bayern Munich, he added: “We know we are able to do it. We showed in the week we had some good performances. We have to leave this behind but talk about it.
"We need to be consistent we can play against many opponents a very good game we are able to do it but you have to do it every day so to get it right and that demands a certain standard so we have to go to higher standards to deliver it."