United were first to confirm the deal on Twitter with a video of Sanchez playing "Glory Glory Man United" on the piano.
Earlier, the Chile worldwide posted a video on Instagram, apparently showing him walking across an airstrip towards a private jet following reports that he was due in north-west England for a medical. "I have spent three-and-a-half wonderful years at Arsenal and I bring with me very positive memories of that great club and its fans", Sanchez said about the transfer, via the United's official press release.
"Wage parity is very important to players".
A picture that was posted on social media on Sunday evening appeared to show him taking a selfie in a United shirt with his name and No. 7 on the back - the shirt worn by club legends George Best, Eric Cantona and Cristiano Ronaldo.
Armenia global Mkhitaryan is closing in on a switch to the Emirates Stadium from Manchester United as part of a deal that will see Sanchez move in the opposite direction.
Burnley have now gone eight games without a win.
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It is believed most of the formalities have now been finalised with the renewal of work permits one of the last issues to be resolved before the transfers of both players are completed.
"I'm very happy that we could finish this deal and I'm very happy to be here", said Mkhitaryan.
It's good that we finally have confirmation of this long-running story.
He added it was a "pleasure" to manage Sanchez, the player he signed for Barcelona from Udinese in July 2011.
"If Man City got him, that would be them pulling ahead and proving they are the biggest side in Manchester at this moment in time, and maybe going forward for the next couple of years.""There is still a big gap and we need to make sure we win our games because if we don't then there's no point even talking about it".
"I've been with the first team every day and I see how Alexis is".
Yet having run United close at Turf Moor, goalkeeper Nick Pope is confident their fortunes will change if they maintain their good performances. "That's all the past and I'm not really interested", he said.