The Ajax manager admits that he may have gone to a club that has "better foundations" in place for his next role.
"I had chances to work at a different club, with a better foundation," Ten Hag told De Telegraaf.
"But I chose United. Things have to get addressed there and that's a challenge. The club and I are on the same wavelength about how I want to do this."
Ten Hag has cancelled plans to join Ajax on their post-season tour of Curacao due to start work early with United.
"It's time to turn the switch now," said Ten Hag, following Ajax's final game of the campaign, which finished level against Vitesse. "It's something that is needed because there is a lot to take care of and I have the energy for it.
"Tomorrow there will be a party with the (Ajax) staff, but I am not going to Curacao. I think that's understandable. There is a lot of work to do there (United). That is the case with every club."
He added: "A number of things still need to be organised in the staff and also in the selection [at Man United]. We will be working intensively on this from tomorrow."
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