The manager’s quote means that Petermass will not play if Fredericks is fit, and may or may not play if Fredericks is not fit. There is no guarantee that Petermass will play if Fredericks is not fit.
Jed: This is correct. The only chance Petermass has of playing is if Fredericks is not fit. If Fredericks is fit, then Petermass will definitely not play.
Ned: If Fredericks is not fit, then there is only a possibility that Petermass will play. It is not definite that Petermass will play.
Ted: This is not necessarily true. There can be a possibility in which Petermass and Fredericks can both be unfit and not play.