UFC Champ Stipe Miocic -- I'm Down to Fight Brock Lesnar ... He'll Lose. (VIDEO)

If Brock Lesnar wants a second UFC comeback fight, he may already have a taker -- heavyweight champ Stipe Miocic ... who says he's absolutely down to rumble with "The Next Big Thing."

We spoke with Miocic moments ago who told us he's excited to see Lesnar get back in the octagon -- but expressed concern over the fact BL's been out of the fight game for a couple years and could be rusty.

But if Brock crushes Mark Hunt at UFC 200 and wants a shot at the title -- Miocic says bring it on ... though he's pretty damn confident things won't work out well for Lesnar.

By the way, Miocic already fought Hunt in 2015 and kicked his ass.