United announced on Monday evening that they had signed Sanchez from Arsenal in a swap deal with midfielder Henrikh Mkhitaryan.
Sanchez has signed a four-and-a-half year deal with the club, while Arsenal announced that Mkhitaryan had joined the club “on a long-term deal”.
The Chile international took to Instagram to thank Arsenal and their fans, but also made sure to criticise former Arsenal players who have questioned his commitment to the club over the past few months.
“There are people (former club players) who have spoken with no knowledge of what happens inside the club and cause damage,” he wrote.
“I must say I always gave 100%, until the last day, when I asked to the Mister to be in the team, because I wanted to be a contribution.”
Sanchez also revealed that he looked to Thierry Henry for inspiration in deciding to quit the Emirates. “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,” he added.
Sanchez leaves Arsenal after four successful seasons, during which he scored 80 goals for the club in 165 appearances.
And he thanked everybody at the club before leaving for Old Trafford.
“I want to say thanks to the technical staff, to the medical team and all teammates with whom I shared many nice things for the club,” he wrote.
“And especially those people who do not see themselves on the covers, but without them nothing would be possible, which are there to prepare food for us and take care of us day by day, those who keep our shoes clean and the grass in the best conditions.
“Many thanks to you for helping us to improve every day. Thank you very much.
“All we achieved and the good moments that I gave to the club, I want to dedicate it to the fans, they are the most important. Thanks for every time you sing Alexis Sanchez baby.”