The 20-time English champions are believed to have agreed personal terms with Sanchez with Arsenal manager Arsene Wenger admitting in a press conference on Thursday [19 January] morning he expects the deal to be confirmed within the next 24 hours. Henrikh Mkhitaryan will move in the other direction, joining the Gunners in a straight swap deal just 18 months on from his arrival in Manchester.
United face a deadline of noon on Friday to register Sanchez in their Premier League squad if they wish for him to feature in Saturday's match and according to the Manchester Evening News, United are pushing hard to get that through in time.
The club even hope to have the deal as soon as Thursday night so Sanchez can join up with his new teammates at Carrington and train ahead of the club's trip to Turf Moor - where the Chile international scored a late winner for Arsenal last November.
While Wenger has not ruled out the possibility of the deal falling through and Sanchez even featuring for the Gunners when they host Crystal Palace on Saturday afternoon, he is expecting the deal to be wrapped up soon.
"Yes it is likely to happen," Wenger told reporters on Friday. "But any minute things can break down. That's how the transfer market is. These things are never guaranteed."
Asked if Sanchez could still play for Arsenal again, he added: "Yes. If it doesn't happen he will play on Saturday," before suggesting a deal with Mkhitaryan is in place. "I like the player. We played many times against him when he was at Dortmund. He appreciated the quality of our games. The wages would not be a problem."
While both transfers are imminent, both Sanchez and Mkhitaryan trained with their respective clubs this morning.