Mourinho has been the most successful United manager of the post-Sir Alex Ferguson era in terms of silverware but his side finished sixth in the Premier League last term and trail leaders Manchester City by 12 points this season.
"I think he goes to a great club and gets a great contract so you can understand it when a professional player can combine both aspects".
The United manager's previous deal was set to expire at the end of next season. I lost a fantastic player; Arsene Wenger lost a fantastic player. To have a player of worldwide level, a reference player of Alexis' calibre right now in a great club is wonderful. "It was a great deal for everyone".
And now Arsenal continue to be dismissed in Chile, not only by the local media, but now by the national team manager as well.
"You go and analyse the numbers and Manchester City spent more money than us, Chelsea spent more money than us, I think even Everton spent more money than us".
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"Could he have performed better for us? They might not like finding themselves on the bench some days but I am happy with that". You have to respect your opponent but not too much - they are human like us.
The boss - who was Real Madrid's manager for two seasons while Alexis was at arch-rivals Barcelona between 2011 and 2013 - has always been an admirer of the 29-year-old and feels he is the flawless addition to a squad already packed with quality.
Jose on @Alexis_Sanchez: "We've got one of the best attacking players in the world".
Arsenal are also looking to add Pierre-Emerick Aubameyang to their ranks this January as they bid to strengthen for the second half of the season and push for a top-four position.
Mourinho, meanwhile, spoke of his delight to extend his stay at Old Trafford, where many expect him to break his tradition of short-term spells at clubs. I was in Italy with him when he was at Udinese and I at Inter.
Arsenal legend, Thierry Henry, has told midfielder, Mesut Ozil to make the team his own following Alexis Sanchez's departure. "I repeat, to be Manchester United manager is an honour every day and I am really happy".