The forward moved to Old Trafford in January 2018, and managed just five goals in 45 matches before joining Inter Milan on loan last summer.
And after leaving United to Inter on a permanent deal, Sanchez decided to have his say on his disappointing spell in Manchester.
In a shocking Instagram video, Sanchez said: "I want to tell you about my period at United, about many things that were said and that made me look bad.
"I got the opportunity to go to United and it seemed tempting to me. It was something nice for me, because when I was a kid I liked that club a lot.
"I ended up signing without much information about what was happening in the move. The first days that I was with my colleagues, sometimes there are things that you do not realise until you arrive.
"The first training I had I realised many things. home and I told my representative: "Can't the contract be terminated to return to Arsenal?
"They start laughing and I told them that something did not sit right with me. It had already been signed."
He said: "In a game with West Ham I wasn't picked. That had never happened to me as a player. It bothered me and I said it couldn't be possible.
"To go from being one of the best in the Premier League to not playing in five months. I came to my house and I was very sad. The next day I trained in a double shift, because I love what I do.
"Then the next coach arrived, the current one, and I decided to talk to him. I told him that I needed to take a breather, and the opportunity to go to Inter. He told me that yes, there was no problem."
Sanchez claimed he was the victim of unfair criticism from people outside of the club.
But the 31-year-old ended the video by thanking his former club.
"I only have words of thanks to United for giving me the opportunity to defend the shirt," said Sanchez. "It bothered me that things did not go the way I wanted.
"If the atmosphere had been more positive, it would have turned out differently. I tell it all now because I have already passed a period of learning as a player and a person. I would have liked to win everything."
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