After conceding two goals early in the second half to fall 3-0 down, the Portuguese coach feels a lack of belief allowed the Ligue 1 side to take full advantage in the 5-2 victory
Villas-Boas' side trailed 3-0 early in the second half in Monaco on Saturday after a first-half goal to Andrea Raggi, plus strikes just after the break from Lucas Ocampos and Radamel Falcao.
Harry Kane opened Tottenham's account in the 57th minute, before Andros Townsend scored the final goal of the contest six minutes from full-time.
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"We had a good meeting at half-time and the problem was that we conceded the two goals straight away.
"We wanted to avoid that because the players left the dressing room with the spirit that we could turn the game around and go on to win it. So we paid a price for that and it made it difficult for the second half.
"Monaco looked strong with individual quality, we made some mistakes and they punished us.
"We had belief that we could have got a better result but we'll now work hard to improve before the season starts.
"We were missing lots of players and they will help us improve but we wanted to do better. We're aware we're better than this but we'll take our lessons from it."
Meanwhile, Villas-Boas confirmed striker Jermain Defoe was substituted early as a precaution after feeling tightness in his hamstring.
Tottenham host Espanyol on Saturday in their final pre-season encounter before their Premier League season opener Crystal Palace on Sunday, August 18.