Schlein has tested positive for Covid and is forced to isolate which leaves Parrys International Wolves a man down.
If they win the meeting it would seal a top spot finish in the regular Premiership season and hand them choice of opposition in the play-offs.
Club owner Chris Van Straaten said: “This is a major blow for the team, it’s out biggest meeting of the season and we’ve lost our captain.
“However, we must also send our best wishes to Rory and his family and hope he feels fit and well very soon.
“We are going to look at our options for a guest replacement now and make a decision on how we plug the gap from there.
“It’s going to be a really tough meeting against the very experienced Peterborough side, they have been excellent all season.
“It’s going to be extremely tense and I know Peterborough will be coming here looking for yet another away win.
“We’re going to need our fans to turn out in force and get right behind us. We need them to tell their friends to come along and enjoy a really great night of speedway.”