Pre-season games usually don't have any motivation for the top clubs to win the game. Yes, I agree that in this match both teams gave one hundred percent of their abilities. But this is not the first time a top team from the English league has lost to a mediocre team. I think Manchester United really made this a game to test a lot of different tactics. Rosenborg scored a goal late in the game so I think both teams played very well. But a defeat is still a defeat and Ten Hag also said that this was not the performance United should have.
As far as I Know, most big teams don't finding motivation in such preseason game's as this could bring injuries to star player's which could cost them big but it's also a great opportunity to try out new signins and new tactics for managers if they choose to make good from this!
As of the loss, I had to look at which senior players featured for Manchester United and I can confidently report that very few of Uniteds big names featured here and we shouldn't put much pressure on them as the result is on the youngsters who failed to deliver a win for lack of team chemistry, but come August ,Utd will not have excuses for failing to deliver wins as the league will be in full swing!!
That why is good to be good if assume Manchester United did well last season many will have considered them the way you thought, even I personally didn't condemn them because I have the same thoughts as yours, likewise I consider the opponent to be playing well better then them because it a privilege to them for playing with a huge team like Manchester United and also most of the players there really want to take advantage of that to make themselves popular for other big teams to see how they are performing to get lift up. It even too early to judge, let see how the senior men will play when the league starts.