United posted a statement on their club website around 9.45am on Friday morning confirming Phelan as assistant.
They stated that Phelan has signed a three-year contract despite speculation that he may take up a director role.
However it appears the news was released early with the report time-stamped at 10am.
It was swiftly taken down before being put back on the website at 10am.
Phelan had been linked with a technical director role at Manchester United as the club look to reshuffle from top to bottom.
It’s believed executive vice-chairman Ed Woodward is keen to have someone who knows the philosophy of the club in the director role.
But Phelan, who returned to the club as assistant in December, will now stay in the dugout.
Phelan said: “I am thrilled to be back as Assistant Manager.
“Manchester United has been a huge part of my life since I joined as a player in 1989 and I am delighted to be able to continue that relationship in this key role.
”Since returning in December, I have enjoyed working with Ole, Michael, Kieran, Mark and Emilio and we are all focused on preparing for the new season.“
Phelan was Sir Alex Ferguson’s assistant for a number of years before leaving in 2013 shortly after the Scot retired.
Current United manager Ole Gunnar Solskjaer, who brought Phelan back to the club in December, was believed to be keen to keep his assistant in the dugout.
And the Norwegian is looking forward to continuing his relationship with Phelan.
He said: ”He walks around Carrington and players really respect him.
“They know what he can do and he’s worked with the best.
“I’m a young manager, I still feel I’m young anyway, and Mick’s got more experience in a few areas than I have.
“I’ve been a player under [Mick] and his knowledge, his experience, his demeanour; everyone in the club enjoys speaking to him, talking to him, discussing football with him.”