The 21-year-old picked up the knock in his side's friendly match against 2. Bundesliga outfit Bochum on Saturday, and was forced to leave the pitch.
Mlapa underwent further examination after the game to determine the exact extent of his injury, and he has now been diagnosed with torn ligaments in his right ankle.
"Injuries are unfortunately a part of the game. The timing and extent of the injury are obviously a big blow, but I have to look toward the future. I'll do my utmost to return as soon as possible," the attacker told Sport1.
Mlapa had only just joined Gladbach from Hoffenheim, and has yet to make his first official appearance for the Fohlen.