The Real Madrid star recently dropped in on the Irish UFC featherweight champion as he prepared for his UFC 202 fight against Nate Diaz on Saturday, and the pair were photographed squaring off:
McGregor, 28, praised Ronaldo's athleticism but said if they were ever to fight for real, it would be no contest. He also joked that the photographs of the pair flattered the Portuguese forward, per BT Sport:
He didn't get in and spar but he's a nice guy and he's in some shape. But there's no competition, I'd slap the head off him.
He's in tremendous shape. He's a true athlete and a great guy, a good friend of mine. I've been talking to him a long time.
He messaged me a long time ago on Instagram a good while back because I was wearing his underwear for the [Diego] Brandao fight. ...
He anticipated the lighting [for the photo], he was under the lighting and had the abs going, he had the pose. I went more lats and he went more abs.
Undoubtedly, McGregor would have no problem seeing off Ronaldo, 31, despite the 2016 UEFA Euro 2016 winner being famously fast, athletic and well-built.
Indeed, Bleacher Report UK previously speculated that an MMA fight between the two would not last long:
Conversely, though, McGregor would have little chance against the three-time Ballon d'Or winner in any kind of footballing contest.
Both athletes are at the top of their respective sports, and McGregor will be aiming to avenge his previous defeat to Diaz at Las Vegas' T-Mobile Arena on Saturday. If he does, he'll claim his first win in the welterweight division.
Back in March, the Dubliner lost to Diaz at welterweight during UFC 198. The Californian had come in as a last-minute replacement opponent for McGregor following then-lightweight-champion Rafael Dos Anjos' withdrawal through injury.
It was The Notorious' first defeat in the UFC, but he can re-establish his fearsome reputation by taking Diaz down in the rematch.