But the Tottenham manager has no plans to drop the right-back because he inhaled the legal gas.
Last weekend a Sunday newspaper published pictures of Walker inhaling nitrous oxide - a legal but dangerous high - earlier this summer.
The gas, also known as "hippy crack", is widely available and legal, but can cause respiratory problems and even death.
Walker spoke to Villas-Boas about the issue earlier this summer and he promised the Tottenham manager that he would not do it again.
"The player has assumed the responsibility of having failed to set an example, which I think is something that is honourable, but you wouldn't expect him to do it again," Villas-Boas said.
"He accepted the responsibility of having failed as a role model towards young people and I don't think we will see the incident again.
"He has assured me before (that he won't do it again)."
Villas-Boas also played down title talk and said finishing in the top four remained the club's main target for the Premier League season.
"Our objective is the same, it's Champions League qualification," he told a news conference ahead of Saturday's home game against Norwich City.
"Our level of expenditure in the transfer market raises expectations but they have to be real expectations, not false expectations," added the Portuguese.
"We don't have the experience to deal with the pressure of a team that plays for the title. We have never done it, you know, in these recent years and we will compete as usual for Champions League places..."
Tottenham finished fifth in the league last season and missed out on a Champions League place to North London rivals Arsenal.
They are currently sixth, with six points from their three matches after losing 1-0 at Arsenal this month.
They have sold Wales winger Gareth Bale to Real Madrid for a world record 100 million euros (£85.3 million) and re-invested the money in a list of new signings, including Argentina forward Erik Lamela from Roma for a club record 35 million euros.
Villas-Boas said it might be too early for the 21-year-old to start on Saturday.
"He is a player with tremendous potential that we have to work on," he said, pointing out that Lamela had limited English and still had to adapt to his new surroundings.
"We are pretty sure he will be able to do (that) but we are unsure of how much time it will take," he said. "The fact that he has yet to train with the team puts him in a very difficult position to start the game but not impossible."