Harry Wilson and Rhian Brewster scored for the European champions but goals from Paco Alcacer, Thomas Delaney and Jacob Bruun Larsen were enough for Dortmund to take a deserved 3-2 win at the University of Notre Dame.
The game was played in temperatures of 33C - in a ground hosting its first football match.
"You could see that Dortmund are a step further in their preparations but there was a lot of good things for us as well," he said.
"How we conceded goals was a lack of organisation, but it is very difficult to have that at this stage.
"Really I learned nothing. But it is difficult when the ball isn't rolling. It was a tough night for both teams - it was really warm.
"I don't like what I see in a game in pre-season very often but there is always a reason for it. I am not over the moon with our performance but I know there is lots to come."