At 154lbs I pick a prime pre-Trinidad Ferocious Fernando Vargas. Sure, McCallum might be winning in the scorecards, but Vargas would stop him around round 7 or 8.
The only way that Vargas could stop McCallum is by throwing salt in his eyes, then having his manager hit Mike with a chair. Even then Vargas would probably have to come off the top rope with an elbow. If none of those things happened, McCallum would beat Vargas into a gargoyle. Probably not stop him, I don't think.
Vargas would be game and have his moments but McCallum is too damned good, tough and skilled. The Bodysnatcher stops him late after a brutal clinic. The last few rounds would get ugly. I watched some McCallum fights at 154 as a refresher and I forgot how badly he busted up folks with that jab and straight right. Nando would look like a gargoyle at the end.
Vargas would have moments and win a couple of rounds. But McCallum is just too tough, too good. Vargas isn’t super quick handed. And he’s gonna be there to be hit. McCallum wins a wide UD or gets a late stoppage.