Think to put some of this in simple terms a reasonably sized chunk of FAIRING inside the exhaust area broke away. If one looks at a very simple example look at the A320 series engines and compare the CFM 56 nacelle and the IAE V2500 one is very short and one is long . The engines are a similar size. So the loss of some of the nacelle doesnt have a major effect on the physical operation in this area. I'M sure if the crew had sight of the damage they wouldnt have carried on but , the damage was not visible so the only choice is to go with what you can see , a good set of engine parameters and a few status messages.