Melbourne confirmed yesterday they had "formally" turned down a bid from BVB for the young shot-stopper.
Muscat told Radio SEN this afternoon: "There's no doubt the boy is distraught and upset.
"It's obviously a huge opportunity when you think he's a young lad from Bundaberg.
"I did say a few weeks ago that the club would struggle to hold onto young Mitch."
However, Muscat says he spoke to Langerak ahead of tonight's Asian Champions League qualifier Beijing Guoan and is confident the youngster "will be focused and turn in a solid performance".