Barcelona have been disappointing in the Champions League for two seasons. The only serious problem Xavi is dealing with is this now actually. Because they didn't have much problem on winning the La Liga title. But they just can't deal with big teams in the Champions League properly. This season they had a really terrible group stage experience. I didn't like Lewandowski's performance in big matches here as well. It was like there was a completely different striker than the one who scores so many goals in the La Liga.
Xavi needs to find a way to consolidate the game plan more for next season. Because the board would ask him to do more than this. Maybe tiki-taka isn't the right choice against such teams here anymore so Xavi had better think about a new formula.
Barcelona want to apply the same plan they play in La Liga in the Champions League which has resulted in them being a failed team every season in the Champions League. Barcelona's main focus should be on the Champions League as they have not won the Champions League trophy for several years. The only way to achieve European excellence is to win the Champions League trophy.
Since Xavi has been appointed as the manager of Barcelona, Xavi should adopt the plan that is needed to win the Champions League trophy and if some players need to be changed to do well in the Champions League, Barcelona should change those players.
Barcelona is now a much more consolidated team after being managed by Xavi,sure it took a bit of time but I think this year Barcelona will pass the group stages and go far,the players also have harmonized between them in a way to play much better than in the beginning so I think with these players they already have,they make a good strong contending team.Of course any addition especially in the midfield which is the most important part of any team as they win or lose you games is more than welcome and I am sure Xavi is already thinking which player to get to fit his needs.