The Gunners forward is set to complete his move to the Red Devils in the next few hours.
Sanchez has arrived at United’s training ground to undergo his medical.
And Henrikh Mkhitaryan will complete a move to Arsenal in a straight swap agreement between the clubs.
Pires also believes Sanchez ultimately wanted to stay at the Emirates.
He told La Tercera: “No, he is not a mercenary. Martin is my friend, but I don’t think you can say that.
Alexis Sanchez arrives at Carrington for Manchester United medical
ZENPIX
“What Alexis wanted was for Arsenal to spend money on other players”
Arsenal hero Robert Pires
“That’s football, and if City does not want Sanchez anymore, it’s their problem.
“And if United wants him and is willing to spend a lot of money, good for him. But for me, Alexis wanted to stay at Arsenal.
“I have not talked to him, but what I see, what I smell, is that he was very good in London, very good in Arsenal.
GETTY
“What Alexis wanted was for Arsenal to spend money on other players.”
Pires also opened up on what the Sanchez deal means for Arsenal and Manchester United.
He explained: “Arsenal loses a great footballer. With a lot of quality and, above all, a winner.
GETTY
“It is the most important, always want to win. So, if tomorrow he goes to United, they will be very happy there.”
Arsenal return to action on Wednesday when they take on Chelsea in the second leg of their Carabao Cup semi-final with the aggregate score 0-0.