The 21-year-old midfielder told Chelsea TV: "I used to come to Chelsea to watch, I used to sit in The Shed a lot. I used to love watching good players - I used to love watching Dennis Wise and before that Kerry Dixon.
"I settled at West Ham because I was most comfortable there and I had a good time there.
"Now my career has come full turn and I'm back at Chelsea. I'm really looking forward to it - it's a great club to be at and it's a great challenge."