I'm not sure I see anyone stopping him. Lots of big bangers at 126 we could name, but I recall Azumah Nelson shrugging his shoulders after fighting him, when asked why he didn't stop the gritty Mexican. He could only say "that's as hard as I can hit."
I think Pacquiao with his speed and power combo could mess up Villasana pretty bad, although Villasana would be fighting till the end, and it would be the ref's call to stop it