"Are we reluctant to get rid of Sonny? Of course we are, because he's a heck of a good rugby player. Our job as coaches and selectors is to pick the best team every week and that's based on what we see in training and what we see in performances, attitude and all that."
Foster insists the second five-eighth is improving.
"When you look at the growth of his game, his defence work has been outstanding.
"Probably what is just missing is the Sonny Bill finesse on the offload. Again, he is improving in that area. Is he the finished product? No, he's not. Is he progressing? Yes, he is."
The Achilles injury Williams suffered while on sevens duty at the Rio Olympics in August last year - which kept him out of action for eight months - hasn't helped his game, Foster said.
"He's obviously had injuries, so we've basically been trying to give him a good run and I think that's been important for his confidence and his growth to get some starts.
"This is a great year for us to do that. Ryan [Crotty], Anton and Ngani played huge minutes in Super Rugby, while Sonny missed most of Super Rugby. So this is a great opportunity to give him a run of starts."
Foster doesn't expect the criticism to get to the code-hopping superstar.
"When you've been around for a while you learn not to put guys in little cubby-holes and say 'that's them'. It's easy for people to critique a player [on] where he's at right now and say 'he's not doing the job, so get rid of him'.
"But these are top athletes, and Sonny's one of a squad, that focus on how they're going to get better and better. He's getting better, but there's still some areas for him to improve. As long as he has that attitude that he wants to get better, then we're excited."
•Full interview with Foster on Newstalk ZB from midday.
Ole Gong for icons of rugby
A week ago, the All Blacks were crowned kings of Southern Hemisphere rugby. Now they'll be rubbing shoulders with Spanish royalty.
A delegation including Grant Fox, Steve Hansen, Jordie Barrett, Keven Mealamu and Conrad Smith are off to Oviedo to receive the prestigious Princess of Asturias Award for Sport, one of the most eminent distinctions in the Spanish-speaking world.
"It probably caught us by surprise when we found out that we'd won it, as we didn't even know we'd been considered," assistant head coach Ian Foster said.
"We've all had to learn about it, to be fair. But it's a great honour, not only for the current team but also previous teams in the history of the All Blacks and what they stand for."
The All Blacks won the award for having become a worldwide icon in rugby, according to the award's jury and will be honoured at a star-studded ceremony hosted by King Felipe IV and Queen Letizia on Saturday (NZT).
"It [the award] reflects on rugby as a whole in this country, and the way the country goes about it. It's an award the whole country can be very proud of."
Previous Asturias winners include Bob Dylan, Nelson Mandela, Al Gore, Rafael Nadal and Michael Schumacher.
