Adrian signed a three-year deal with the Hammers yesterday.
"Leaving Betis has been the hardest decision of my career. Here I have developed as a player and as a person since I was 10. My thanks to all the people who have helped me in this time. Thanks to my teammates and (coach Pepe) Mel, who took a very brave decision putting me in goal.
"I thank the Betis fans for their love and there can be no doubt that the feeling is mutual.
"This decision is super important to my life and it was not easy. I do not feel I have betrayed anyone. Football is fun and takes many turns. For years I've been at the club and given everything. This is not a goodbye, it's a see you later and I hope to return to my home someday."