Monday morning quarterback

One who criticizes or passes judgment from a position of hindsight.

A person who criticizes or passes judgment from a position of hindsight, as in Ethel was a Monday-morning quarterback about all the personnel changes in her department--she always claimed to have known what was going to happen. This expression, first recorded in 1932, alludes to fans who verbally "replay" Sunday's football game the next day, the quarterback being the team member who calls the plays.