But his newest challenger, Evangelista "Cyborg" Santos? He's okay, and there seems to be one simple reason why the Strikeforce welterweight champ can be familiar with his upcoming opponent and yet not have anything at all bad to say about him.
"He doesn't really speak English," Diaz told MMA Fighting. "I've met him. He's cool. I've got respect for him. But I mean, he doesn't really speak English so it's not like we've had a bunch of conversations."
Friendships have been built on frailer ground.
In a slight departure from his recent outings, Diaz predicted that this fight will feature "no bad blood, just a lot of blood," and he may be more right than he knows.
Santos has at times seemed almost incapable of moving backwards in a fight, preferring instead to charge straight at his opponents and stay in their faces until they wilt from the pressure or find a way to make him stop.
While Diaz said he feels like he understands Cyborg's style, the hard part for him has been finding sparring partners who can mimic it and still show up for training the next day.
"Guys who fight like him don't make it into the gym that much because they get hurt a lot," said Diaz. "Those guys, if we spar and work out hard a couple days in a row, they don't show up for a few days. Especially guys like him, they try to be explosive and powerful and lift weights and all that to make up for the technical aspects of things. I think sometimes that works against them. Sometimes it works for him, but it's not very consistent."
At the same time, Diaz has to respect Santos' power. His style might leave him open to counters, and he's not exactly known as one of the more elusive fighters in the division, but if he lands you're going to feel it.
Diaz seems well aware of that fact, and sounds as if he's in no hurry to trade punches with the Brazilian challenger.
"The thing is, if you take shots from this guy, they're going to be hard shots. If I lose this fight, I'm going to the hospital. That's how I feel about this. It doesn't change the way I look at it. He's a good fighter."
The one thing Diaz feels supremely confident in is his assertion that Santos will want to avoid being on his back at all costs – so much so that Diaz said he doesn't expect to see much in the way of kicking or Thai clinching from him. With his size advantage and his superior jiu-jitsu, he said, Santos will be too wary of being taken down and kept there.
"He can't go on bottom against me," said Diaz. "If he goes on bottom, I'll wreck him. I'll finish him. He doesn't want to be there. His only chance is to move around and throw punches, which is what I do. He really does want to come out and do something crazy in the first or second round. I just have to avoid that."
Does that mean Diaz is looking to win this one in the championship rounds, perhaps test Santos' cardio a bit? Possibly. But he's not ruling out other options, he said.
"It could go either way. He could really screw up and I could finish, or he could end up getting beat up the whole fight. Or I could go in the hospital immediately."