http://www.independent.co.uk/sport/football/premier-league/inside-city-football-group-manchester-citys-network-of-clubs-new-york-melbourne-girona-a7934436.html?amp
This may be of interest to you all
FA rules do not allow loan players to play against parent clubs to avoid any smell of corruption of match fixing.
Roberts played for Celtic against City in the Champions League as that is under UEFA therefore corruption is expected.