The impulse drive on the Miranda, Constitution, and Constellation class are essentially visually identical. The Excelsior has two domes, but two identical domes that are still clearly part of the impulse drive. I see no compelling reason to assume the circular structure on the underside of the Reliant that is visually disimillar to the impulse dome and also not directly connected to the impulse drive serves the same function because it just so happens to also be a circle. We know the Enterprise was able to restore partial main power after suffering significantly more serious damage, so I see no reason to assume that under other cicumstances that the damage to the Reliant would be crippling. Especially given that the Reliant isn't crippled by it, or even damaged comparitvely to the Enterprise.