Matip and Sane were together at Bayern's Bundesliga rivals Schalke before both left for the Premier League in the summer of 2016.
“He is a top player," the defender told Die Welt. “Unfortunately, he will give Bayern a lot of joy."
Elaborating on his "unfortunate" point, he added: “Well, he won't make it any easier for the competition, which already has a hard time keeping up with Bayern.
"Leroy is a player who can help a team enormously."