1999 Chevy Lumina Oil pump?

I just put a used engine in my 99 chevy lumina. The engine ran fine when it was in the other car. In my lumina I get an oil warning light. My manual says its most likely the oil pump. Is it possible to damage an oil pump while swapping out engines? Could it just be a sensor? I looked in the valve cover and dont see any oil ciculating. Also there was a green wire coming off the rear head that wasnt on my original engine. Could this be the cause and what is it???? There dont seem to be any place that this wire plugs in. It looks like some sort of sensor. Thanks for any info.
Teagen