Rangnick has told ESPN that he would like to keep the youngster, but admitted that the Toffees will have the final say on the matter.
"If it were down to us, I'd already know the answer," he said. "But it's not only down to us. We have no clause in the contract.
"I think that we can only answer the question once the season's finished. It also depends on what happens at Everton."
Lookman has scored two goals in seven appearances for the Bundesliga outfit since joining them on loan in January.