EXCLUSIVE

Ryan Seacrest is the lifeblood of "American Idol" ... and without him, the show would struggle to survive -- so says executive producer Nigel Lythgoe.

Nigel was out in L.A. yesterday when we asked if Seacrest was considered dispensable, much like the judges -- and Nigel replied, "Ryan Seacrest drives the entire program. He's not on an earpiece or anything. He's not beholden to anybody. He controls the show. He's really important."

He added, "It would be very, very difficult to replace Ryan Seacrest."

But the bottom line, Nigel believes "American Idol" transcends any one individual -- saying, "I've never believed that Idol as a franchise is beholden to any individual because everybody said it would die the minute Simon Cowell left, and it hasn't."

Perhaps, but it's hardly the same either ... so we gotta ask ...