After the Group G tie, won 3-0 by the visitors, a spokesman said the English Football Association had reported the incident to European football's governing body UEFA.
"We've raised it with the Bulgarian Football Federation and the UEFA delegate," the FA spokesman told reporters. "That's all we're saying about it."
Former Germany playmaker Matthaus said: "I didn't hear it but it is pitiful for such things to happen. I would like to apologise on behalf of the federation."
Manchester United winger Young played for 62 minutes before being replaced by James Milner.
It is not the first time a black player from an English team has been targeted in Bulgaria.
In 2005 CSKA Sofia supporters were heard aiming monkey noises at Liverpool's French striker Djibril Cisse during a Champions League match.
‘Chelsea have to qualify for the Champions League for this season to be a success – they showed that they have the players to perform at a high level, they need a win quickly now’ Blues legend’s fears over recent bad run
'I’d probably pick Cole Palmer alongside Bellingham and Rice in there at the moment. For me, Declan Rice is good enough to play that central midfield role on his own' Thomas Tuchel advised to DROP Manchester City star by England legend
