The 20-year-old signed a new five-year contract with the Blues before joining Slaven Bilic's newly promoted side on a season-long loan on Thursday.
"There was a lot of interest from the Premier League, which is where I wanted to play this season, but this is where I want to be," he said.
"West Brom stood out to me because of the way they played last season. I played against them two or three times and I liked how they approached the game.
"I felt that it would suit me and it would be best for my development. I felt it could help me become the player I want to be and I'm pleased to get this agreement sorted."