The Mexican ace faces Brit star Rocky Fielding in New York on Saturday night.
Alvarez is the hot favourite having edged past Gennady Golovkin in his previous fight.
Khan – who lost to Alvarez in 2016 – reckons the 28-year-old is uneatable at the moment.
The Bolton lad reckons unbeaten 41-year-old Mayweather would now struggle against his old foe.
“I like Rocky, he’s a brilliant guy but I’ve been in with Canelo and I just can’t give Rocky a chance,” Khan told Sky Sports.
“Canelo is a beast.
“The way he beat Triple G, I just think no-one can stop him.
“Even if Mayweather came back and took a rematch with him, I think he would beat Mayweather.”
Fielding, 31, says he wants to avoid any scoring controversy with Alvarez.
Speaking ahead of the fight, he said: “I haven’t looked into that.
“As I say, I’m not looking – I can’t think about judges and other things.
“I’ve just gotta go in there and do what I’ve gotta do, and box.
“And that’s all I’m thinking about.
“That’s it, really.
“Judging, yeah, I’m not really putting me energy into it.
“I’m just thinking of going in there.
“I know what I’m up against and what’s in front of me, and I’ve just gotta be the best I can be and see where it goes.”
Alvarez is nursing a cut under his right eye following a feisty sparring session.
However. he assured reporters that it wouldn’t affect his performance.
“Just a little scratch,” Alvarez said when quizzed about the injury.