Juan Mata bid farewell to Chelsea's fans in an emotional open letter on his website on Sunday.
The Spanish attacking midfielder concluded his record £37.1 million (HK$475 million) transfer from Chelsea to Manchester United on Saturday after completing a medical.
"I won't forget the faith you put in me when I was given the player of the year award the last two seasons. I feel really proud and I will never forget those nights," Mata wrote.
"Thank you so much for the way you have treated me. You have been unbelievable. I hope you understand that after going through a complicated and, for me, new situation in the last few months, I believe this is the best choice to carry on with my career and keep enjoying this sport."
He alluded to difficulties that he had with Chelsea manager Jose Mourinho, but still thanked the Portuguese.
"There've been plenty of good moments: the FA Cup, my first trophy as a Blue, the immense happiness of winning titles for the first time in the club's history, such as that magical Champions League night in Munich and the Europa League," he said.
"Every message I got through the social media, every nice word you have said to me on the street, and of course, your support in the stadium, has made me feel incredibly proud of the Chelsea fans.
"To Jose, for making me a stronger player because in difficult moments, I've learnt to remain positive."
United manager David Moyes said Mata would be the first of many new signings this year.
"I want the quality of Juan to be the start of it," Moyes told United's TV channel MUTV.