Emergency - Inground Liner Install, a return gasket on the deep end fell off behind liner during fill. Am I in trouble?

Do you have another one? You need a gasket.
Aside from that, Other than you probably noticing where it fell for as long you have that liner it shouldn’t be a problem.
 
Sometimes there will be a cork one if you buy a new return fitting - it goes behind the liner (between the liner & the wall), the rubber one goes on top (between liner & fitting plate)
 
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