Sportal.co.nz 22.May.2013Getty Images
Cooper was left out of Robbie Deans' initial 25-man squad for next month's British and Irish Lions series on Sunday.
The controversial 38-Test first-five can still force his way into the squad with a strong showing for the Reds in their tour match against the Lions on June 8.
But Williams, a World Cup-winning All Black, admits he was stunned to learn of Cooper's omission on Sunday.
"Quade's one of my real good mates and to me he's the best number 10 in this country by a long shot," he said.
"There's some good players, talented players but that's just my opinion.
"Selectors and the powers-that-be have other opinions. We'll see how it goes, but I'm happy to see Izzy (Israel Folau) get in there."
Williams, who accused three boxing judges of corruption after his other good mate, Anthony Mundine, lost a unanimous points decision to Daniel Geale in January, was reluctant to comment any further when pressed on Cooper's axing.
But the 27-year-old couldn't help himself when reminded of Deans' criticisms of Cooper's defensive game on Sunday.
"I don't want to get involved in it. I'll probably stay out of it this time. Whatever I say seems to get me in a bit of trouble," he said.
"It seems to me there's a lot of finger pointing saying he can't do this and he can't do this. I spoke to him and said 'just be patient' let his talent do the talking.
"It's pretty tough to come back from a knee reconstruction but the last month he's been on fire and he's only going to get better."
Williams also backed former rugby league superstar Folau to make a big impact in the series if selected.
"It'll be good to see him out there," he said.
"I'll be like everyone else, waiting to see how he goes. He's done really well so far. It's exciting to see him play.
"I said a couple of weeks ago, he's one of those guys that just has that thing that you can't really coach.
"I wish I had that, just being able to beat players with ease like that."
When finally asked a question about his own future, the off-contract Roosters star replied: "I'm not sure (what I'm doing next year).
"I say it every year whenever I get asked. I just make sure I do what I do and that's play footy well, control what's happening on the field and if I do that well enough then things off the field will fall into place."