The 20-year old has been charged with engaging in a sexual act with a minor and suspended from playing with either the Sky Blues or the Australian U20 team.
He lodged an appeal to allow him to have his suspension overturned but an independent disciplinary committee ruled that the A-League ban would stand and he would be available for national team selection until late Octiober meaning he will miss the U20 World Cup.
"We believe very strongly in the presumption of innocence until proven guilty, and Sebastian has not been found guilty of any wrongdoing at this point," said chief executive of the Professional Footballers' Association, Brendan Schwab.
"We're disappointed that he will not be able to represent Australia at the Under-20 World Cup because he's clearly an outstanding player. Based on the support he has received from the PFA, Sydney FC, Melbourne Victory, the media and the national teams set-up, he is also clearly a player of outstanding character."