The Catalan giants finished 13 points behind La Liga champions Real Madrid, their campaign ending in a whimper on Sunday with a 2-0 home defeat to Villarreal.
Barcelona were beaten in four of their last nine games in all competitions.
They were on a plane the next morning for their first-ever match in Australia, a friendly against an A-League All-Star team coached by former Manchester United striker Dwight Yorke and captained by former Manchester City defender Jack Rodwell.
Xavi has brought 20 players to Sydney, including captain Sergio Busquets, Dani Alves, Pierre-Emerick Aubameyang, Frenkie de Jong, Memphis Depay and teenage sensation Ansu Fati.
Gerard Pique and teenage forward Pedri were among those missing.
“Next season we need to improve a lot, we improved a lot since November,” said Barcelona midfield legend Xavi, who was appointed in November with the team lying ninth.
While he helped revive them and they achieved the most important objective of qualifying for the Champions League, they went out to Frankfurt in the Europa League quarter-finals and in the last 16 of the Copa del Rey.
“I think we are in a good way, we have to be positive. But next season, at least, we need to compete and win the Champions League, why not,” said Xavi.
“We are rebuilding the team. We have a plan, of course, and we need to be competitive next season. We cannot permit another season without winning titles.”
Prolific Bayern Munich forward Lewandowski, who was top-scorer in the Bundesliga last season for the fifth year running, has been heavily linked with Barcelona.
His agent Pini Zahavi told German daily Bild this week the striker wants to leave Bayern, with Barcelona reportedly offering 32 million euros ($34 million) to sign the 33-year-old.
Xavi refused to confirm his interest when asked by AFP.
“I cannot confirm Lewandowski or any other names,” he said. “Of course we are working hard and it is obvious that we need to improve.
“It’s a matter of time,” he added of unveiling new players. “If we have to announce something we will announce it, but it is not the moment right now.”
Barcelona squad for All Stars match: Marc-Andre ter Stegen, Ronald Araujo, Sergio Busquets (capt), Ousmane Dembele, Dani Alves, Memphis Depay, Ansu Fati, Adama Traore, Luuk de Jong, Jordi Alba, Frenkie de Jong, Samuel Umtiti, Pierre-Emerick Aubameyang, Gavi, Alejandro Balde, Alvaro Sanz, Arnau Tenas, Mika Marmol, Jandro Orellana, Anontio Aranda.