The 20-year-old scored 17 goals in 23 games for the Baggies before returning to Stamford Bridge this summer.
"I think I developed my skills, playing in small spaces, big spaces and playing back to goal," Lukaku told the club's official website. "I am quicker and stronger, I improved overall and that is a good sign. Hopefully I can continue like that in the next years.
"Steve Clarke put me on a platform where I could shine and he changed my life, because now the impact I have on some people is a bit bigger. I am very happy that I worked under him because he taught me a lot about the game - the mental aspect and little points I had to improve on, he really took the time with me and I will thank him for all my life."
Lukaku joined Chelsea from Anderlecht in summer 2010 for around £10m.