Blackadder today said Williams, who comes off contract with the NZRU after this year's World Cup and is reportedly being chased by rugby and league clubs around the world, was "just as important" to New Zealand rugby as Carter and McCaw.
Carter last week signed a four-year contract extension with the NZRU which gives him the option of time away from New Zealand rugby during the contract term, while McCaw is expected to pen a similarly flexible deal this week.
"He's just been inspirational," Blackadder told Radio Sport today when asked about Williams, who is also continuing his boxing career while contracted to the NZRU.
"He's played some fantastic rugby for the Crusaders and when you look at just how important he is for the World Cup and beyond, I just think he's one of those players that is just getting better and better, strength to strength and he's a role model.
"He's a little different because we're talking about him and his boxing so he creates a lot of interest but he's one of those guys who has x-factor all over him and he can just create and do things that other guys just can't do.
"It's absolutely critical that the NZRU re-signs him and he stays with New Zealand rugby for another couple of years."
Blackadder felt that Williams, who played two tests for the All Blacks late last year and has been one of the standout players in the Super 15, should be allowed to take part in a few boxing fights each year as part of his contract -- if that's what the player wished to do -- to "supplement his income".
"For the sake of one boxing fight or two boxing fights a year, I think it's a great trade off to keep him in New Zealand," Blackadder said.
"It's just common sense. It's better to do that than say 'we can't afford him' and then he goes overseas and plays his rugby in Europe."
The competition's leading offloader, who has recovered from a knee injury and is likely to play in Sunday's match against the table-topping Reds in Brisbane, is set to take part in his fourth professional boxing bout on June 5 - during the Crusaders' bye week - when he fights Alipate Liava'a, of Tonga.
"The boxing will go ahead as long as he is fit but I sure that would just be common sense," Blackadder said.
"Obviously he'd never put himself at risk. That would have to come down to Sonny Bill and his management team."
