He had spent two seasons away from Chelsea on loan at West Bromwich and Everton where he’d scored 17 goals and 15 goals respectively.
After his signing the youngster spoke to the media and said: “I'm 21, I need to be playing in a good team. I needed to be in a place that felt right.
“I decided very quickly I wanted to come back. This is the place I belong,” said the Belgian national.
Club manager Roberto Martinez was ecstatic with the signing as he said: “This signing is not just important for this season. It is a significant day in the history of this football club.”