Alexis was outstanding on debut in the 4-0 triumph.
"I'm very happy," Mourinho said when questioned on Sanchez's display. "I think no surprise for anyone, three years in the Premier League, playing every week and everybody knows what Alexis is, he's the kind of player that to buy, you don't put a question mark in front, you don't think about adaptation or something like that in relation to the English football.
"So no surprise for us, fantastic player, he shows that for the last three years and the past three days where he was training with us.
"I think he needed to play, he spoke [to me], he needed to play, he didn't play for Arsenal I think the last two or three matches, his training was not good, when you have half of your body in the Emirates and half of your body in Manchester, you don't train well and he needed to play.
"I was not sure about 90 minutes, I wanted to go for the 60, 70 or 75, and was good because people starts understanding his game better and he can also understand his fellow players better.
"I was impressed with his attitude, I think I did also my job well in that I told my players to be ready for that [physical approach], and never react because sometimes the top players lose their head and lose their balance and sometimes they are the ones who are going to be punished."