Matt Law of the Telegraph reported the Chilean superstar will pen a four-and-a-half-year deal to move to Old Trafford, as the Gunners wait on Henrikh Mkhitaryan's decision on whether to join them.
Per Law, the total value of the package to capture Sanchez will cost United £60 million. Manchester City and Chelsea were not prepared to match the Red Devils' financial approach, leaving the player the opportunity to arrive at the Theatre of Dreams.
If Sanchez arrives at United, he will add to an already strong team.
Zlatan Ibrahimovic has struggled to make it back into the first team since a major knee injury last year, and manager Jose Mourinho needs a striker to complement Romelu Lukaku.
Mkhitaryan's decision could be vital to the speed of any transfer, but Gunners legend Ian Wright has criticised Arsenal's stance.
Per Law, Wright said:
"Mkhitaryan is not a player Arsenal have been after—he's a player that Arsenal have been handed.
"It's the same with [Pierre-Emerick] Aubameyang—a couple of years ago, he was on everyone's radar but now he's gone a bit cold. Then all of a sudden, he's on Arsenal's radar—it all feels panicky for me.
"Of course, they are both quality players. But if you are losing Sanchez, you need players that will hit the ground running. I'm not sure about Mkhitaryan's state of mind right now."
United are set to pay a heavy financial price to tempt Sanchez, but as City race away with the Premier League title, Mourinho has to react with aggressiveness and decisiveness.