The 18-year-old successfully completed his medical at Old Trafford on Tuesday.
United could decide to loan the striker to an English club or send him back to his present team Universidad in Chile.
“I am super happy to get to Manchester United. I’m fulfilling a dream,” he said.
“I am going where I wanted. You look on television and see the outstanding players, scoring spectacular goals. I cannot waste this opportunity. It is not clear what will happen with my future.
“I want to play in the first team of Manchester United. That’s my first thought.
“If they decide to send me on loan, I would prefer to continue at Universidad, although I will do whatever United deem appropriate.”
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