Rousey, then, is a gigantic question mark. Nunes is not.
The "Lioness" is the undisputed UFC women's bantamweight champion. The Brazilian fighter is the strongest, fastest and hardest-punching woman in the world besides Cristiane "Cyborg" Justino, who was recently flagged for a potential doping violation after being caught using anabolic steroids in 2012.
Nunes' power was on full display in her last fight against Miesha Tate for the title at UFC 200. Nunes methodically stalked down her more experienced foe, waited for her opening and then landed a surgical onslaught that broke the legendarily tough Tate, who slumped to the mat before giving up her back. Nunes sunk in a rear-naked choke, and just like that, the fight was over. Nunes barely had a scratch on her.
The stunningly easy win made Nunes the third woman to hold the bantamweight title since Rousey lost to Holm. Tate and Holm's downfalls combined with Justino's PED controversies and Nunes' current four-fight win streak mean today's fight was the only logical bout the UFC could make for Rousey's return.
After so much time off, it's a bit of an anticlimax. But Rousey is the greatest female fighter of all time, and Nunes is by far the best 135-pounder who has fought in 2016.
Here is what each fighter needs to do to win:
RONDA ROUSEY
Rousey was a ridiculously dominant 12-0 heading into the Holm fight. She was more athletic, stronger and had better hips than anyone she fought.
Unlike the vast majority of MMA fighters, who start as strikers, wrestlers or jiu-jitsu practitioners, the 29-year-old Rousey is first and foremost a judoka. The martial art at the Olympic level is mostly about leverage and torque.
Rousey's "Mortal Combat"-style finishing move - her hip toss into an armbar - is a perfect example of judo's particular physicality.
In MMA fights, when attackers gets inside, they'll normally try to dirty box, Muay Thai clinch or wrestle their opponent to the ground.
Rousey doesn't do any of that. She locks up her foe's arms in the exact opposite way a wrestler would, then uses her hips and butt to launch her victim into the air.
When she pulls it off, Rousey lands on top of her opponent. Confusion usually follows, with limbs flying in every direction, and this is where Rousey truly shines. She moves explosively on the ground and looks to end fights with brutal arm bars.
That is what Rousey will try to do against Nunes, who is a far superior striker. Eating a couple of Nunes' punches to get inside, Rousey will try to control the Brazilian's body and take her for a ride.
If she can do it early, that's great for Rousey, but she also could look to prey upon Nunes' one big weakness: her conditioning.
If the fight gets into the championship rounds, Rousey's experience and endurance should prove decisive.
AMANDA NUNES
While Rousey will look to get on the inside and create chaos, Nunes will try to keep things upright.
The Brazilian has always been lethal on her feet, but the 28-year-old Nunes' new-found control is particularly dangerous because, more than Rousey, she is a complete MMA fighter.
She's a black belt in jiu-jitsu, has solid traditional take-down defence and has the looseness of a great striker. The quality that brings it all together is her ferocity.
Nunes will not wear down Rousey by peppering her face with boxing-style jabs like Holm did. She'll try to control range while maintaining pressure, wait for Rousey to make a mistake, then pounce with fast and accurate punch combinations.
If she lands enough clean blows, Nunes has the power to knock out Rousey, but there is a downside to her explosive style: She could wear herself out, like she did against Cat Zingano.
Nunes has improved dramatically since that fight, her only loss in the UFC. Working with her longtime girlfriend and training partner, Nina Ansaroff, who is also a psychiatrist, Nunes learned how to control her emotions inside the Octagon, according to a lengthy profile by MMAFighting.com.
The results have been devastating.
If she can keep her head and handle the pressure of the moment, Nunes has everything she needs to knock out the previously unbeatable Rousey.
