The 19-year-old striker's season-long loan deal from Chelsea is due to expire in the summer.
Clarke now accepts that an extension of the Belgian's temporary contract is unlikely.
"I'd love him to stay with us but we have to prepare for life without him," the Scot told reporters. "He likes it here and he's been nominated for [PFA] Young Player of the Year which tells you that he has been successful with us."
Lukaku has scored 13 goals in all competitions for West Brom this season.