Naming Allen the backup also means Alex Smith very likely will be inactive for Week 1 against Philadelphia.
Washington traded a fifth-round pick for Allen earlier this year and Rivera repeatedly claimed there would be an open competition between Allen and Haskins for the QB job in Washington.
That never materialized as COVID-19 forced the NFL to skip all preseason games, and Haskins responded to Rivera's challenge for better leadership.
It makes sense for Allen to be the backup, as he knows Washington offensive coordinator Scott Turner's system and has started 13 games over the last two seasons for Rivera in Carolina.
It also makes sense for Smith to be listed as the third-string quarterback.
Smith's comeback story has already reached epic proportions just by making the active roster, and plenty of people still don't expect the 36-year-old to actually play this season.
He's already come back further than many expected just by making the Washington 53-man roster, though watching Smith's return to football, it would be silly to dismiss any future prospects for the former No. 1 overall pick.
For Week 1, Haskins is the starter and Allen is the backup.
If 2020 has taught us anything, don't make many predictions more than a week out. Things can change quickly this year.