The saga of the January transfer window is expected to be over by Monday, with the Chilean set to arrive at Carrington on Sunday morning before undergoing a medical later on in the day.
As Sánchez comes, Armenian midfielder Henrikh Mkhitaryan will move in the opposite direction and is understood to have reluctantly agreed terms with Arsenal.
Mourinho confident of completion
Speaking after watching his Manchester United side battle to a 1-0 win against Burnley at Turf Moor, courtesy of Anthony Martial's second half strike, José Mourinho finally confirmed speculation.
"I expect [an announcement] soon or never," he said. "I think it is so close, so close, so close that if it does not happen it is not going to happen, so I am positive. I know my people are doing everything, absolutely everything they can.
He stated that no personal contact has been made with Alexis Sánchez yet, but is confident that his higher-ups will secure a deal after snatching him out of Manchester City's grasp earlier on in the month.
"The owners with the green light and Mr [Ed] Woodward working hard, I think everybody is giving everything they can and I think they are going to be successful."
Sánchez brings needed competition
The arrival of one of the most potent attackers in the Premier League is bound to put his players on notice and Mourinho thinks he'll bring crucial competition for offensive places, something currently limited at the club.
"You know, in this moment there is no competition for places. In this moment we have four attacking players for three positions; three are starting, one is on the bench and Zlatan [Ibrahimović], the whole season, was not available.
"You look at the central defenders; we play Jones and Smalling, Rojo was on the bench and Victor [Lindelöf] in the stands, Eric Bailly is injured. You look at the midfield; Fellaini on the bench, Herrera on the bench, McTominay on the bench, Carrick in the stands.
"And you look at the attacking areas and we need that extra competitive internal competition, but also solutions for us, so let us see."
United return to training on Tuesday, when the new recruit will meet his teammates and manager for the first time, in preparation for the FA Cup fourth round trip to Yeovil Town on Friday night.