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)."
Walker spoke publicly of his regret at inhaling the gas last weekend and he also apologised to England manager Roy Hodgson.
Hodgson accepted the apology and played the Spurs right-back in the two games against Moldova and Ukraine.
Villas-Boas has no doubts about selecting the 23-year-old for Spurs' game against Norwich on Saturday.
"He seems in the right state of mind," the Portuguese added.
"He told me he was in the right state of mind for England as well, so I see no issue there."