With Sanchez having agreed terms and successfully passed a medical, that unveiling is now expected to happen on either Monday or Tuesday.
#MUFC #AFC confirmation of Sanchez/Mkhitaryan swap Mon/Tue as work permits need updating. Arsenal stepping up Aubameyang pursuit - confident but will drag as must agree fee & possibility of player going other way to #BVB If done no significant funds available for further signings— David Ornstein (@bbcsport_david) January 21, 2018
According to BBC's David Ornstein, the last detail that requires attention before the deal can be fully signed off relates to the formality of an update of the 29-year-old's work permit.
As expected, Sanchez was wearing the number seven shirt on his back when he was snapped from a distance by somebody who happened to be inside Old Trafford at the same time.
He is the first player to wear the famous number for United since Memphis Depay was sold last January. Michael Owen, Antonio Valencia and Angel Di Maria are the only other players to have occupied the shirt since Cristiano Ronaldo left in the summer of 2009.
Sanchez's arrival will offer a huge boost to United over the second half of the season. Not only is he a proven Premier League star after three fine seasons with Arsenal between 2014 and 2017, he is also eligible to play in both the Champions League and FA Cup.
The Gunners were of course absent from the Champions League group stage, while Sanchez was omitted from the team that was humiliated by Championship side Nottingham Forest earlier this month, meaning he is not cup-tied in either competition.