The 22-year-old forward suffered medial ligament damage in his right knee during Alex's recent friendly with Manchester United and is set for a lengthy spell on the sidelines.
"It is not good news concerning Mathias. He has a medial knee ligament injury and it will be months rather than weeks with him," manager Steve Davis told the club's official website.
"We will see how he is once the bruising settles down but it doesn't look good. It was a bad challenge because the full-back has left the ground and followed through. Pogs should have avoided being involved in the tackle but he went into it and it was risky.
"It is difficult to put an exact time span on it but it will be a month or two. Rob (Sharp) has weaved his magic in the past and hopefully he can knock some weeks off of it. We'll have to wait and see.
"It is a blow for us but we have to get on with it. It is a blow for Pogs because he has only just come back from his thigh operation.
"He is a bubbly character and I've spoken to him about it. He has had some really bad luck but we will keep his spirits up and he is okay."