Why did Rayleigh think he's too weak to beat Blackbeard?

[Warmmedown]

Based on what? Hearing of some previous Blackbeard fight?

 

[Symbiote]

He's like 90.

 

[Future Warrior]

Sensing his Haki and knowing he has two overpowered DFs.

 

[sei'taer]

Blackbeard built a strong reputation during the timeskip. Raleigh isn't stupid, nor is he young.

 

[SSJ2]

Blackbeard beat Marco pretty easily during the 2 year gap. Rayleigh isn't far from that level at his age, but with much worse stamina.