The reason for Dorrans' withdrawal has not been disclosed but fellow midfielder Morrison injured his groin during Albion's Barclays Premier League defeat to Swansea on Sunday and was substituted at half-time as the Baggies went down 2-0 at the Hawthorns.
Afterwards Baggies manager Steve Clarke said: "He felt a pain in his groin just before half-time, the last couple of minutes in the first half.
"The medical people will assess it but I would imagine he is doubtful for the Scotland trip."
It is not known if either player will be fit for the trip to Macedonia that comes four days after the Belgium match.
Scotland lie fifth in Group A with five points from seven games.