Chile forward Sanchez, 29, who almost joined Manchester City last summer, has signed a four-and-a-half-year deal.

City decided earlier this month that they would not pursue their interest in him as they felt making him their best-paid player may affect team morale.

Mkhitaryan, 29, moves in the opposite direction to Sanchez after scoring 13 goals in 63 appearances for United.

The Armenia international moved to Old Trafford from Borussia Dortmund for £26.3m in July 2016.

He has made only 10 Premier League starts this season and it is understood there was a breakdown in trust with Mourinho, who has made Sanchez his first signing of the January transfer window.