Cigano's thrilled for his former opponent ... and as a matter of fact, he wouldn't mind a rematch in the squared circle sometime down the road.

TMZ Sports talked to 39-year-old dos Santos earlier this week ahead of his fight with Alan Belcher (more on that soon) ... when we asked Junior about Francis' move to the boxing ring.

"I'm so happy for [Ngannou]," Junior told us, explaining ... "All the boxing guys are always talking, saying on the striking side we are not able to go there and put on a good performance against them."

Junior believes Francis proved the opposite against Tyson Fury ... MMA fighters can hang with the very best when it comes to striking with just hands.

"Francis Ngannou was fighting probably the biggest heavyweight in the history who never lost a fight," JDS explained, adding, "and he did what he did. I was there. That was so amazing to see that. And, it showed what kind of work we [MMA fighters] do."

Now, Francis has another massive challenge in front of him in the form of Anthony Joshua next month ... and Junior is clearly cautioning folks not to count him out.

FYI, JDS is well aware of what Francis can do. The two stars fought back in June 2019 ... but after a flurry of punches, the fight was stopped just 1:11 seconds into the first round, with Ngannou winning by TKO.

Junior wouldn't mind running things back!

"That would be amazing, too! I think actually that would be something very good, not just for me, but for him, as well."

For dos Santos, any fight with Ngannou would be down the line. First, he's got a big-time matchup with 39-year-old Belcher, who is coming off a decision win over Roy Nelson at Gamebred Fighting Championship 6, Jorge Masvidal's bare-knuckle MMA promotion.