Villas-Boas needs his striker back in form quick and he thinks the player will not be affected by the criticism that has come his way in recent weeks.
"I'm not sure if it bothers him," the 35-year-old said.
"Players build their own motivations. Perceptions and media opinions can influence the public in general, but I think the player strives on his own motivation in the end.
"I think (he's working as hard).
"The injuries he had and the African Nations Cup took him out for lots of games and Defoe's form this year had been magnificent."