Mauricio Pochettino bade an emotional farewell to United States soccer on Friday, describing his decision to join Manchester United as "one of the most difficult" of his career.
Pochettino, 54, spoke to media in the United States just hours after being officially named as Manchester United's new manager following the sacking of Michael Carrick.
The Argentine, who in August signed a four-year contract extension to continue as US coach through to the 2030 World Cup, fought back tears as he drew a line under his successful two-year reign.
"We learned a lot from you all, from this amazing country," Pochettino said. "I think this was one of the most difficult decisions of (my) career."
Pochettino will remain in his post to oversee the United States' back-to-back CONCACAF Nations League matches against Haiti on Saturday and next Tuesday before starting with the English Premier League giants.
The former Tottenham, Chelsea and Paris Saint-Germain boss became emotional again as he reflected on his stint in the United States, which saw him lead his team to the last 16 of this year's World Cup.
"To the staff and the players, I want to say thank you so much for the last two years," Pochettino said. "You made me feel really happy sharing an amazing project. It's difficult now for me to finish.
"We were very excited to share a project which we really believed. It makes the decision harder.
"It's difficult to talk now because I'm a very emotional guy. In some part it's a very sad decision, in another part we are professional and we are trying to make our dreams come true."
Pochettino, who has been linked with Manchester United throughout his career, said negotiations to seal the switch had unfolded rapidly.
"All was so quick because we arrived here last Friday and Sunday afternoon, this type of situation started to appear," Pochettino said.
"We were talking with the federation and yesterday, when the agreement was reached, we had a meeting and I communicated with the players. They were very supportive.
"US Soccer and Manchester United agreed we would continue, and that will not change our commitment."
Pochettino said he had not considered a possible return to the Premier League when he signed his contract extension with US Soccer in August -- but added that the possibility of joining Manchester United was too big to turn down.
"I never said I wanted to go back in the Premier League," he said.
"Not too many opportunities can happen in your career with a side like Manchester United. That is difficult to turn down when it arrives.
"But I want to make clear that when we decided to extend, it was a commitment for the federation."