I watched the game.
Utd played like a team that haven't played together before and are playing their first game of the season. Rosenborg played like a team who are at close to peak fitness and wanted to make a good showing against a big name team. They were quicker to every ball, were more aggressive and had better shape and movement. It's to be expected from a pre season friendly, the reason you go and play these teams who are mid season is for the challenge of playing against players who are already up to speed, rather than playing another team who are going to be going through the motions of pre season. It gives players and coaches a good idea of where they need to be at over the coming weeks for the more challenging pre season games.
To be honest, Utd were poor, very sloppy and maybe could have done better with one or two chances. Vitek made some really good saves and stopped it from being what could have been a fairly embarrassing result. But, they should really have been reduced to 10 men after about 20 mins, horrible tackle on Casemiro.
But overall, there's been a huge over reaction to the result on here, it's a pre season game where about 5 first team squad players actually played, the rest were just kids getting a taste of the action to see if they have what it takes to be involved in the upcoming games in America.