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Hearn, 20, had recently forced his way back into first team contention after missing most of the season with various injury problems and was a possible contender for the substitute's bench in Cardiff.
He now joins Tony Warner, Andy Roberts and Kevin Muscat on the sidelines.
Lions assistant boss Ray Wilkins confirmed: "There's no chance for Tony Warner and Andy Roberts to play in the final. They're both out of it.
"It's always a blow for them as people. We worked hard to progress and when you miss out it's hard to take.
"Charley will also miss the game with a hernia. It's a great shame because it's a once in a lifetime opportunity, just as it was for Kevin Muscat and Danny Dichio."