Sibierski, 33, has not trained all week and looks set to miss Saturday's game with Birmingham, adding even more woe to an already poor frontline.
It does however seem likely that both Antonio Valencia and Mario Melchiot will be ready for the match..
Manager Chris Hutchings said: "Antonio is a quality player - he likes taking people on and creates chances. He links the play well and can be a very influential player for us.
"We couldn't risk him against Portsmouth after his knee swelled up on inter-national duty. But he did a bit of training last Friday and has stepped things up this week."