Most predators will scavenge if they are given the opportunity, so it's not an either/or argument. I tend to think of the T-Rex as the analog of the Great White Shark. He cruises the periphery of the pack dino, ambushing an individual animal for a chunk of meat. After the victim has bled-out, he returns to scavenge the carcass (assuming other T-Rex's don't drive him/her away. Pack-hunting is a possibility, but I don't know if there's much evidence for it.
"Pack-hunting is a possibility, but I don't know if there's much evidence for it."
I agree that T-Rex probably was both a predator and a scavenger. As far as pack-hunting, what were the majority of potential prey animals in North America when T-Rex lived? There were thousands and thousands of Hadrasaurs, which migrated from North Western Canada south into the US, and back again.