Name 1 time a rematch was given for an underdog fighter? Has an underdog fighter ever lost and been given an immediate rematch?
Anderson was given a rematch. But he was expected to win that fight. It shocked the world. He was even favored in the second fight.
The reason why champions are given immediate rematches, is because they are favored to win and it is shocking when they lose.
Even if it was an upset, GSP wasn't given an immediate rematch after the matt Sara loss.
There really hasn't been too many immediate rematches. It's not all that common or likely. 