Britain's Mirror newspaper said Sanchez was due to arrive in Manchester with agent Fernando Felicevich, with a medical scheduled for Sunday afternoon, while Sky reported that Mkhitaryan would have a medical in London. But he also aimed a dig at pundits who have questioned his commitment in recent weeks.
Sanchez had claimed in his first interview as a Manchester United player that he always admired the club and even spoke to former boss Sir Alex Ferguson over the possibility of playing under the legendary manager.
The 29-year-old Chile striker, who came close to joining Manchester City last summer, has signed what is believed to be a four-and-a-half-year contract that will make him the highest-paid player in the Premier League.
United have been linked with moves for Antoine Griezmann, Gareth Bale, Christian Pulisic and Paulo Dybala but Sanchez's arrival has put them on the back burner for now.
Retired defender Silvestre played for both United and Arsenal over the course of his 18-year career, but is best remembered for his successful spell at Old Trafford where he won five Premier League titles and lifted the Champions League.
Meanwhile, Mkhitaryan moves in the opposite direction on a "long-term contract" after failing to establish himself in Mourinho's side.
Armenia global winger Mkhitaryan, also 29, will head to the Emirates after he fell out of favour with United manager Jose Mourinho during 18 months at Old Trafford.
"When a player of that ability comes along, I think you got to be in the market for him", Irwin, who played over 500 games for United during 12 trophy-laden seasons, told Reuters.
Sanchez nearly joined City at the end of the August transfer window but that deal fell through as Arsenal manager Arsene Wenger was unable to sign a replacement in time. "That can help us as well because he can play together with Ozil as well as playmaker, so overall it's a good move", he added.
Arsene Wenger left the Chilean out of Arsenal's last two matches away at Bournemouth and at home to Crystal Palace. "Probably not. You never know in football but they are in a very, very strong position".
Prior to that he had been expected to join Manchester City, who had a deal worth up to £60m accepted for Sanchez last August but could not complete a deal on transfer deadline day.