Skipper Schlein has tested positive for Covid and the club will share his four programmed rides around the rest of the team.
Bosses asked Ipswich star Danny King to step in but the former British Champion is unavailable.
Parrys International Wolves Promoter Chris Van Straaten said: “Peterborough are a tough enough prospect with a full team, never mind when we are a man down.
“We asked Danny who is unavailable and we have made the decision to use the rider replacement facility.
“We’ll take a win by whatever margin, any win will do. If we can do that we can pick our opposition for the semi-finals.
“Ironically Peterborough are in the same position as they are using the same facility for Ulrich Ostergaard and they’ve been doing pretty well going in that direction.
“It’s a huge meeting and as I said on Friday when we got the news about Rory we’re going to need all our fans right behind us to be the seventh rider.
“Peterborough are a great team, they’ve been consistent all season and their experience has helped them immensely.
“They’ll be a tough proposition for our boys but this is the time for us to show what we’re made of.”
Peterborough have also drafted in Drew Kemp as a replacement for their injured reserve Jordan Palin who is out with a collarbone injury.
Wolverhampton: Sam Masters, Luke Becker, Nick Morris, Ryan Douglas, Rory Schlein R/R, Broc Nicol, Leon Flint.
Peterborough: Ulrich Ostergaard R/R, Chris Harris, Michael Palm Toft, Scott Nicholls, Bjarne Pedersen, Drew Kemp, Hans Andersen.