Former midfielder Butt has taken charge of the club's Under-19 team and is hoping to have the 17-year-old available for selection in the UEFA Youth League at a later date once the paperwork for his arrival from FC Zurich is processed.
"Saidy is not available at the moment because we're waiting for international clearance," Butt told ManUtd.com. "It could take time, even weeks or months. We're not sure yet.
"The important thing with Saidy was getting him signed up and then training to improve his fitness up. He has got massive potential."
Butt and Warren Joyce spotted Janko during the Blue Stars FIFA Youth Cup in May when he impressed in the 0-0 draw against the Reds and helped FC Zurich win the tournament without conceding a single goal.
"He played against us in the Blue Stars tournament and stood out straight away," added Butt.
"Saidy played against us in the Blue Stars tournament in Switzerland and stood out straight away."
- Nicky Butt, United Under-19s coach