Rasmus Hojlund struck early to give United the lead, before the Gunners hit back through Gabriel Jesus and Gabriel Martinelli.
Hojlund and Leny Yoro were both forced with injury scares in the first-half.
Ten Hag later said, "I think it was a very good level for the third week of pre-season, from both sides, but in the first half I thought we were the better team. We scored a very good goal, a very good ball in behind from (Marcus) Rashford and then Hojlund with a very good move and a finish, I was very pleased.
"In such occasions he is very strong and he is a threat for every defence and he has scoring abilities."
"It was different, it was a mixed team and this was the first time they have played together. We had a player playing full-back for the first time in his life (James Scanlon) and then you have to play against experienced Premier League players, that is not easy. I think at the second goal we let him down a little bit and we could help him there by giving him a little bit more support by supporting or covering. Overall I think the team played a good performance and we could have drawn this game."
On Hojlund and Yoro's substitutions, he added: "Of course it is too short and we have to wait over 24 hours, then we will hopefully know more. We were careful especially with Leny as he only did 50 per cent of the sessions. He has to come up but let's be positive and see what comes out."