armchair quarterback
noun
a person who offers advice or an opinion on something in which they have no expertise or involvement
Examples
Super Bowl parties are full of armchair quarterbacks.
The year before Bret Favre and the Packers won the Super Bowl, I knew a co-worker who was obsessed with the idea that Favre was too wild a passer and that the Packers should bench him for another QB. He was pretty quiet after Favre and the Packers won the Super Bowl in '97.