Wilhelm Steenkamp and Dean Greyling will not be considered for the Bulls' opening Super Rugby match of the season, against the Stormers on Friday.
The two regular squad members are suffering from an AC joint injury (Steenkamp) and stress fracture to the foot (Greyling).
According to Bulls team doctor Org Strauss neither will be able to play at Loftus Versfeld this Friday.
Steenkamp will still be out for another two weeks, while Greyling should be able to train next Monday.
This leaves Marcel van der Merwe as the only long-term injury casualty, with the former Cheetahs prop recovering from a knee-operation.
He should be back in action in May.
Dawie Steyn also started training this week, following his leg fracture, but will not be match fit yet.
Strauss said that a couple of players who did not train on Monday - including Ulrich Beyers, Akona Ndungane, Jacques du Plessis, Jacques Potgieter and Louis Fouché - will all be considered for selection.
Bulls coach Frans Ludeke will name his side on Tuesday.