Aberdeen midfielder Jack suffered a head knock in training and has been withdrawn as a precaution.
It is expected that he will be fit for Sunday when the Dons travel to Easter Road, however.
Peter Pawlett is still in the Scotland Under-21 squad and could feature at some point this evening.
The UEFA European Under-21 Championship Qualifier between Scotland and Bulgaria takes place tonight, Monday 5th September 2011, kick-off 7.30pm, at St. Mirren Park, Paisley.
There is a cash gate in operation, with entry prices at £5 for adults and £2 for children.