We reported late last night (Per PWInsider) that John Cena vs. Samoa Joe was originally planned for last night's Raw, however those plans obviously changed as the match didn't take place on the live broadcast.
According to PWInsider the reason for the change was due to the fact Samoa Joe has suffered some sort of knee injury during the WWE Live Events over the weekend. Joe went up against Cena on Friday and Saturday but was absent from Sunday's show where Cena worked against Bray Wyatt. There is some concern that Joe could be out of action for several weeks.
WWE has yet to confirm this injury, but we'll keep you updated.