Sanches has endured a horror season on-loan at Swansea City.
"He [Renato Sanches] is going to come back and Niko Kovac [Bayern's new coach] is going to try to bring his whole strength back. It will be an interesting challenge," Rummenigge told Merkur.
Rummenigge is confident that the new coach of the Bavarians, Niko Kovac, will be able to get the best out of the player.
Sanches is tied to Bayern until 2021.