Cardiff City midfielder Rubin Colwill is again unavailable to U21 manager Paul Bodin, selected in Wales’ senior squad for a friendly against Finland and World Cup qualifiers against Belarus and Estonia.
Colwill, 19, who made his senior Cardiff debut in February, also missed the previous game against Moldova because he was part of Wales’ 26-man squad for Euro 2020.
The trip to Sofia is the first of four consecutive away games for Bodin’s team.
They are due to travel to Moldova on Friday, 8 October, followed by a trip to the Netherlands on Tuesday, 12 October.
They then visit Gibraltar on Friday, 12 November before a home game against Switzerland four days later.
Wales U21 squad: George Ratcliffe, Nathan Shepperd, Daniel Barden, Billy Sass-Davies, Morgan Boyes, Brandon Cooper, Eddy Jones, Ryan Astley, Owen Beck, Fin Stevens, Niall Huggins, Terry Taylor, Sam Bowen, Sam Pearson, Joe Adams, Luke Jephcott, Elliot Thorpe, Rhys Hughes, Jack Vale, Christian Norton, Ryan Stirk.
