The 19-year-old Belgian has been in fine form for the Baggies this season, notching 13 goals in 26 Premier League appearances.
According to The Mirror, Lukaku would like to remain in the Midlands next season, but Clarke has resigned himself to losing the 6'3" frontman.
"He is Chelsea's player," said Clarke. "He belongs to Chelsea and he will go back there in the summer.
"I have never thought it would be possible Chelsea would let him come back next season. He came on a season-long loan and we're grateful we got him."
Lukaku joined Chelsea from Anderlecht for £18m during the 2011 summer transfer window.