Sanchez is understood to be on the verge of completing his much-publicised switch to Old Trafford, having passed his medical at United's Carrington training ground this afternoon.
The 29-year-old had initially been expected to join Manchester City, but United emerged as frontrunners following reports of a much more lucrative offer for the Chilean, who is out of contract at the end of the season.
Sanchez has been criticised in some quarters for choosing United over the Premier League leaders, but he has now retweeted an article claiming that he will be a "bargain" for the club.
The action is the closest Sanchez has come to publicly commenting on the move, which is expected to be completed within the next 24 hours.
United midfielder Henrikh Mkhitaryan will move in the opposite direction as part of the deal, and Sanchez could make his debut for United when they face Yeovil Town in the FA Cup fourth round on Friday night.