Liverpool continue to be linked with the Guinea international and they had a 75million euro bid rejected earlier this summer.
RB Leipzig are preparing to make their debut in the Champions League this season and they are determined to keep hold of Keita this season.
Reports in Germany claim Keita's contract includes a 50million euro release clause after the 2017-18 season and Rangnick insists the club may struggle to keep him in the future.
He told reporters: "Next season Naby theoretically has the option to leave.
"I presume that he'll play a great season again, above all in the Champions League. And then there will be huge clubs interested in him.
"So probably it will be tough for us to make him stay."