Ajax ended the Eredivisie season with a 2-2 draw at Vitesse on Sunday but Ten Hag, who will replace interim coach Ralf Rangnick, said postmatch that he has a lot to do in order to prepare United ahead of next season and confirmed he will not attend Ajax's postseason party or trip to Curacao.
"There is a lot of work to be done," Ten Hag said. "The job of the manager is to prepare the preseason. There's a lot of work for our preseason.
"There are things that needs to be organised staff and squad-wise. We will work intensively on that from [Monday onwards]."
United are not in action again until Sunday, away to Crystal Palace, but they have lost four of their last seven Premier League games to end the season in a bad way.
Cristiano Ronaldo said earlier in the week that United must give Ten Hag time if he is to succeed.
"What I know about him is he did a fantastic job for Ajax, that he's an experienced coach," Ronaldo said. "But we need to give him time.
"Things need to change the way he wants. I hope we have success, of course, because, if you have success, all of Manchester is going to have success as well. I wish him the best.
"We are happy and excited, not only as players, but the supporters as well. I wish him the best and let's believe that, next year, we are going to win trophies."