Defoe scored twice as Spurs cruised past Dinamo Tbilisi with an 8-0 aggregate victory, while midfielder Lewis Holtby added a third in a comfortable 3-0 second-leg win.
|6/5||Tottenham are 6/5 with Bet365 to finish in the top four this season
He told ITV Sport: "We couldn't be complacent. All the boys did extremely well. After a 5-0 win you can come into the game with a laid back attitude. They didn't.
"They kept their professionalism and got some chances and got some good moments of football with some good combinations.
"Jermain has the goalscoring ability and once again he proved it tonight.
"What we are trying to do is build a strong team with strong players and everyone has to compete for a place.
"We always want to make teams stronger than before so players who are in the top five of Tottenham's all-time goalscorers can only make you stronger."
Villas-Boas was also questioned on Gareth Bale's potential transfer to Real Madrid, though the 35-year-old was non-committal when responding about the Welshman's future.
He added: "No news. This could drag on for a couple of more days. I'm not in control of the situation to give you a definite answer."