UFC's Brian Ortega Offers to Fight Gennady Golovkin, 'I'm Down!'

If Conor McGregor can fight Floyd Mayweather ... why not Brian Ortega vs. Gennady Golovkin?!

The UFC rising star tells TMZ Sports he's all the way down to take Canelo Alvarez's place in the May 5 boxing match ... now that Canelo has withdrawn from the fight.

"I'm down. I'm down. I'm down," Ortega said while out in NYC to promote UFC 223 ... "I want Triple G."

Look, Ortega -- and his boxing coach James Luhrsen -- know it ain't exactly an even fight ... but Ortega says he's down for the challenge and would work his ass off for a shot at one of the best boxers in the world.

"Triple G, I'm a fan ... If you want to fight an MMA fighter and you're looking for a fight, let's do it, man!"