Liverpool were linked with a move for the 22-year-old this transfer window, although it was a move that ultimately failed, but Rangnick believes it is likely that Keita will leave at the end of the season.
The Guinea international has a £48m release clause in his contract which will be activated next year, giving suitors an opening to snap him up.
“Next season, Keita theoretically has the option to leave,” he told Bild.
“I’d like to think that he will play another strong season, also in Champions League, and then there will be big clubs who are interested in him. Therefore it will presumably be pretty tough to keep him here.”