The young fullback arrived from FC Zurich earlier this month.
"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.
"Warren and I watched him a couple of times over there and, when you see a player like him with his physical ability at a young age and ability with a football, it was a no-brainer really.
"We were just lucky with the timing as we were there and he was playing against us and was available to get. Although it was a bit of hard work to do it, we've got him and it's just a no-brainer because he has fantastic potential."