This might change quite a bit with the addition of Gundogan who brings in all the experience necessary to compete in the Champions League. He is also a few years younger although Barcelona isn't really adding strength to their squad. Busquets left, Gundogan joined. They are now considering a swap of de Jong for Silva from Manchester City. This would mean de Jong is gone, Silva is added. There is still no significant change that makes the team fundamentally better.
The midfield has filled up well since Busquets left and Barcelona also have some energetic young midfielders in the team. Gundogan's experience can give Barcelona strength and he can make an impact on possession in midfield. De Jong's exchange with Silva didn't really matter much in my opinion, because de Jong is also a creative midfielder and would make an ideal partnership with Gundogan in midfield.
Barcelona only needs one more senior player at the back and this player must really be a tough defender who is able to break attacks from opponents, perhaps as is typical of their old players who have left this season.
They have a good center line from the offense with Lewandowski via Gundogan in the midfield and ter Stegen in the back. But is it quite the same as it was last season despite some names that will have changed when the new season starts.
Lewandowski, Gundogan and Ter Stegen are players who have the mentality to face crucial matches and these three players can provide encouragement to other young players, hopefully there will be additional senior players in the Barcelona team next season so they can compete in the Champions League with perfect strength on all fronts.