Werner has scored 42 times in 77 appearances for Leipzig since his 2016 arrival from Stuttgart, and a number of European clubs - including Real Madrid - are said to be interested in the 22-year-old.
Rangnick, however, has said that the striker will spend "at least" one more season at the Bundesliga club.
"That's just not an issue for us," Rangnick told Sky Germany. "Timo has made it clear to us as well as in many interviews that he will definitely play for us in the coming year.
"Moreover, he has stated that he wants to play at least one season under Julian Nagelsmann (who joins from Hoffenheim next season).
"With only two years left on his current deal, Leipzig would, however, most likely not allow Werner to go into the 2019-20 season without committing his future to the club for longer than the summer of 2020. We will try to extend his contract ahead of time."
Werner has also managed eight goals in 17 appearances for the Germany national team.