Armenia global winger Mkhitaryan, also 29, fell out of favour with United manager Mourinho after just 18 months at Old Trafford. He is a guy who loves football, and when he is on the football pitch, he always gave the maximum for Arsenal.
Henrik Mkhitaryan seen on camera mouthing 'Yo Pierre, do you want to come here?'
"Since I was a young lad I've always said that my dream was to play for Manchester United, and I'm not just saying that because I'm here now and today it's come true", he told MUTV.
Speaking ahead of the second leg of the Carabao Cup semi-final against Chelsea on Wednesday night, Wenger again defended the decision to allow the forward to join a rival club, insisting that his replacement Henrikh Mkhitaryan would make Arsenal's attack more "efficient" and "integrated".
He was adjudged as the player of the year in the German top flight which couldn't stop Jose Mourinho from asking for his services.
"The players and manager decide what is best for them". A tireless and direct runner with a natural inclination to attack, Sanchez would likely have been a good fit for United under Ferguson, who quit in 2013 after almost 27 trophy-filled years.
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He has played 119 times for Chile and helped his nation win back-to-back Copa America titles in 2015 and 2016, although they failed to qualify for next year's World Cup in Russian Federation.
"The golden destiny of Alexis", said daily newspaper La Tercera, which claimed the news would warm the hearts of Chilean fans still upset over the country's failure to qualify for the World Cup.
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.
"I'm very happy that we could finish this deal and I'm very happy to be here", Mkhitaryan said.
Arsenal manager Arsène Wenger added: "Henrikh is a very complete player".
"He creates chances, he defends well, he can absorb distances and he's very committed as well", Wenger said. However, Sanchez did not use the trick Virgil van Dijk did with Liverpool, stating there is only one club he would play for in Premier League.