The eight-goal leading scorer has been affected by groin and hip problems since scoring the opening goal in the 2-1 win over Arsenal in the Carling Cup final at Wembley.
He has played only 17 minutes since that triumph although he has linked up with his country for their clash with Northern Ireland in Belgrade and next Tuesday's encounter in Estonia.
Blues manager Alex McLeish said: "He went out to the national squad and their people have been having a look at him.
"We know that if he's still not right, he won't play. He is aware you can't afford to take any risks.
"Ziggy was in great form before the injury and we will have to see if he isn't right for Serbia whether or not he will then be okay for us."
Source: PA
Source: PA