Alexis Sanchez has hit out at former Arsenal players who "cause damage" with their media speculation, and insisted he always gave 100 percent for the club.
Since the start of the January transfer window, Sanchez has been linked with an exit from the Emirates Stadium, but it soon emerged that a transfer to Old Trafford - rather than Manchester City - was in the offing for the Chilean global.
He said: "I'm very happy that we could finish this deal and I'm very happy to be here".
He would have been available on a free transfer when his contract expired in four months' time but once City baulked at the amount of money involved in a mid-season deal, United manager Jose Mourinho urged his club to step in.
Arsene Wenger is in very real danger of leaving the club in a worse position than he found it which is downright incredible after all he did for them.
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.
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"Alexis is one of the best attacking players in the world and he will complete our very young and talented group of attacking players", the Portuguese said.
"He will bring his ambition, drive and personality, qualities that make a Manchester United player and a player that makes the team stronger and the supporters proud of their club dimension and prestige".
Wenger admitted that it was a huge blow to Arsenal to see Sanchez join United. Mkhitaryan joined United for around £30 million in 2016 after three years at Borussia Dortmund.
For United, they get one of the Premier League's very best players to pair in attack with Paul Pogba, Romelu Lukaku and Zlatan Ibrahimovic.
"I'm very excited to get a player in who will be committed and focused".
Mkhitaryan struggled to make the grade at Old Trafford, scoring 13 goals and providing 11 assists in 63 appearances after arriving 18 months ago.