Jason Alexander -- I Didn't Know Who Steinbrenner Was ... Before 'Seinfeld' Gig

Much like George Costanza, Jason Alexander had to pretend at a job he was pretending to have.

Alexander was out and about in New York City when our photog tried to chat him up about the New York Yankees, figuring Alexander was a fan from his "Seinfeld" days.

But as it turns out ... Alexander knows NOTHING about sports and says he had no idea who George Steinbrenner even was ... even though he shot several memorable scenes poking fun of the notorious Yankees owner.

In the immortal words of Mr. Costanza ... it's not a lie, if you believe it.