Upon becoming a member of Jose Mourinho's squad, Chilean forward Sanchez, who had been a target for United's rivals Manchester City both in the summer and earlier this window, said: "I am thrilled to be joining the biggest club in the world".
The swap deal sees Armenia global winger Mkhitaryan, also 29, head to the Emirates following a mixed 18 months at Old Trafford had been expected for several days.
I remember today, a conversation I had with Henry, a historic Arsenal player, who changed club for the same reason and today is my turn.
"The players and manager decide what is best for them".
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It was claimed the two clubs had chose to divide the deals to work on them separately, with Sanchez moving to Old Trafford for a fee in the region of £35 million. "We're replacing a very good player with a very good player, a world-class player with a world-class player", said Wenger.
It's never an easy task wearing the boots of Alexis Sanchez, but with Arsenal's new head of recruitment Sven Mislintat showing a lot of faith in him, he won't want to disappoint.
"I can not understand anybody wanting to leave Arsenal".
Arsenal made a decision to cash-in this month rather than lose him for nothing in the summer and United emerged as favourites for his signature after blowing Man City out of the water with a lucrative contract offer.
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. After that, I accept that because we accepted that we would let him go.
United are second in the domestic standings, 12 points behind Manchester City, and will hope Sanchez can help them cut the gap on their neighbours whilst also helping them progress in the Champions League, where they face Sevilla in the last 16.