The forward has signed from Benfica for an undisclosed fee.
Willock will have a COVID-19 test before he can join with the rest of the team for training.
The 22-year-old has represented England at nearly every youth level, and will be hoping to kickstart his career in the Championship.
He said after signing: "I am really excited.
"I can't wait to meet my new team-mates and to get started. I am really happy to be here.
"The team plays good football and I think that will suit me.