The Catalans are eight points clear of rivals Real Madrid at the top of La Liga and beat Los Blancos in Saudi Arabia to win the Spanish Super Cup in January.
Last season, Barcelona failed to win a trophy but the mood changed in the summer when the club brought in several players, including former Bayern Munich striker Lewandowski.
"Lewandowski is accustomed to winning titles with Bayern. He had brought us that winning mentality, that of a champion," Xavi told reporters Saturday.
"In his head, defeat is not an option."
The 34-year-old Polish international has scored 23 goals in 26 games this season for Barca.
Barcelona can extend their lead on Madrid if they win at Villarreal on Sunday, before Carlo Ancelotti's side face Elche midweek, due to their participation in the Club World Cup.
"Each game is key and more so at this stage of the season," added Xavi.
"We are eight points clear, tomorrow we could make that 11."
