Arsenal legend Robert Pires has said he cannot understand why Alexis Sanchez is being allowed to join rivals Manchester United, adding that he believes the player wanted to stay.

Pires, an Arsenal ambassador, spoke out before Sanchez completed his move to United, with Henrikh Mkhitaryan coming the other way.

"In my opinion, Alexis wanted to stay at Arsenal," he told La Tercera newspaper. "I haven't spoken to him, but from what I see and feel he was doing very well in London, very well at Arsenal. What Alexis wanted was for Arsenal to spend money on players. That is my opinion and maybe I'm wrong.

"I know that if I have a player like Sanchez, the first thing I do is to renew his contract. Then I would buy players to make a better team and try to win the league.

"What I don't understand is that a team like Arsenal allows their best men to sign for a rival side."

Sanchez has scored 60 goals and set up 25 in 122 Premier League appearances for the Londoners since his £30 million move from Barcelona in July 2014.

And Pires said: "Arsenal are losing a great footballer. He has a lot of quality, but above all, he is a winner. If he joins United, they will be very happy there."

Alexis Sanchez is set to wrap up a move to Manchester United. Catherine Ivill/Getty Images

The former France international said he did not believe Sanchez had been unhappy at Arsenal and added: "I've been with the first team every day and I see how Alexis is.

"I honestly can say that his behaviour is very good. He is very professional, he always wants to win, even in training.

"It's a way of telling the team: 'If you don't want to be like this, then I leave.' I think this is all Alexis wanted to prove.

"He's impressive in training. He is there, working with a smile, and happy. He has a very positive attitude towards himself and towards Arsenal."

Pires also hit out at claims made by former Arsenal teammate Martin Keown, who described Sanchez as "the biggest mercenary in football."

"He is not a mercenary," he said. "Martin is a friend of mine but I don't think one can say that. If United want Sanchez and are ready to spend a lot of money, that's good for him."

Pires said Arsenal should be grateful for what Sanchez had achieved during his time at the club, adding: "He has always given his body and soul for this team.

"When he signed for Arsenal, he told me: 'I'm here to win tittles.' People will understand how very important he was for the club.

"He is going to be successful [at Unted], just as he was at Barcelona and at Arsenal. I think United are going to be very happy."

Pires said he believed big name players had left Arsenal because of their failure to challenge for the Premier League title in recent years, but backed manager Arsene Wenger.

"The last title was in 2004 and that is a very long time for this club," he said. "It's even difficult for the coach. The club has to sign quality players. The club has a lot of money -- it has to buy.

"We have quality players, but we need to buy more. We need to have two teams just like City and United, like Real Madrid and Bayern."