Willock is not part of Jose Mourinho's first-team plans and will now spend the campaign out on loan.
The 21-year-old made the bench for the Premier League clashes with West Brom and Crystal Palace last season.
But he didn't get a run out and is yet to play for United's senior side.
Willock will be out to pick up experience this term before returning and battling for a spot in the team next year.
The midfielder joined United in 2012.
The Premier League leaders are expected to have a quiet deadline day.
Speaking yesterday, Daily Record reporter Duncan Castles said on The Transfer Window podcast: "I had a chat with my Manchester United contact this morning, he does not expect them to make additions although Jose would still like a left-back or a winger, [Thomas] Lemar being one of the guys he wants they don't expect players to come in at this stage.
"Also interestingly, don't expect players to go out. Andreas Pereira is being chased by Valencia, but Mourinho wants to keep him in the squad, he sees him as a free hit."
