The former Bayer Leverkusen winger was Mourinho's first signing of his second homecoming.
"Everyone knows the club was following him for a long time before I joined.
"And when I was faced with the situation where Andre was not far from being a Chelsea player, I gave the last push because I like the player very much. I think he offers us a lot.
"He's an attacking player who can play in all the attacking positions and a number of attacking systems.
"He has the qualities that modern football demands.
"Andre is creative, fast and has a good shot, he can play on both sides and through the centre.
"He's a multi-functional player in attack and I'm very happy that we have signed him."