The crowd of Liverpool fans are planning to display in Sofia banners saying Michael Shields is innocent.
The BBC also said that, according to sister Laura, Shields would be watching the Liverpool-CSKA clash on television in his prison cell.
Supporters of Europe's champion club who travelled to Sofia for the game are no more than 600, reports say.
Liverpool and CSKA Sofia clash Wednesday night in a Champions League qualifier.
Anticipation for the game has been mixed with controversy over the Shields case.
The teenage Liverpool fan was sentenced in Varna to 15 years in prison. He was found guilty of an attack against a local bartender, although another Briton - Graham Sankey - has confessed to the crime.