In the Further Instructions Episode Locke was told to save Eko. He did in fact save him from the polar bear. But, he did not save Eko in the end from the smoke monster. Someone else pondered that the bear was actually the smoke monster in another thread and that is an interesting possibility. Even though John saved him, he did not recognize that the threat was ongoing. He thought the danger had passed, and moved away from protecting Eko to finding the other losties. Boone had told him there was nothing he could do for them at this time but that he needed to Save Eko. I noticed that when they were heading for the Pearl Hatch, Desmond asked Locke:
So are you going to SAVE Eko, or to try and find the computers?
Almost as though he were trying to remind Locke that he still needed to save Eko. If John had recognized the supernatural nature of Eko's attacker and that the danger was not past.....would he have been able to save Eko last night?