New coach Tata Martino is not changing a lot of things. While Ancelotti is trying to implement a new system, Martino wants to bring back the defensive pressure Barcelona had with Guardiola in charge. For a team that has been playing together for six seasons, Martino was probably right when he decided to keep playing the same 4-3-3 formation.
As always, FC Barcelona is a possession-oriented squad. it will not be possible to keep the ball away from them. Rayo Vallecano managed to do that last month in Vallecas' tiny pitch, and Barcelona still succeeded and won that game 0-4. Messi and his teammates had won it all before traveling to Pamplona to play against Osasuna. There, Barcelona couldn't score a single goal and the match finished with a scoreless draw. Three days later, Barcelona visited AC Milan and could only get a 1-1 draw at San Siro. Two recent draws that might have shown Ancelotti the way to stop Messi and company.
Ancelotti can't afford to be naive against Barcelona. Again, they're going to have the ball, so Ancelotti's project of "spectacular and attacking football" should be put into rest for at least these 90 minutes. With Busquets, Xavi and Iniesta in the midfield, FC Barcelona will surely control the tempo of the game. But even though Xavi has recovered some of his lost groove, Iniesta's form is not great and Busquets will not be able to stop Real Madrid's transition offense by himself. Cesc Fàbregas' workrate in the midfield could also be important if Martino decides to use him, but that would mean one player out of Pedro/Alexis, Iniesta or Neymar will have to sit the game out.
Neymar will attack through Arbeloa's side, and the Spanish defender struggled a lot against him during Brazil's 3-0 win against Spain in the last Confederations Cup. Still, Arbeloa's defensive skill set is always reliable, so Neymar will not have an easy task.
Barcelona's depth in the defensive line could be dangerous for Martino's side. With Piqué struggling to recover from a groin injury -he should be able to play though- and Puyol still coming back, Mascherano or Bartra could get minutes. Again, Piqué will likely play the match, but Real Madrid's pace could be hard to stop if he's not completely ready.
With Jordi Alba out, Brazilian defender Adriano will start in Barcelona's left back. Adriano is a decent defender with a dangerous shot from long range, but he doesn't have Alba's speed, so Arbeloa and Di María's job will be much easier without Alba on the pitch. Dani Alves will be given the task of stopping Real Madrid's most dangeorus flank, the left. He will surely get help from Pedro/Alexis, but Marcelo, Cristiano Ronaldo and Isco/Modric could be too much for him.
And finally, Lionel Messi will try to punish Asier Illarramendi's lack of experience in these kind of games. Xabi Alonso was a defensive specialist that helped Ramos and Pepe a lot against the Argentinian striker, but Illarramendi still has a lot to learn. As always, if Messi is not comfortable, Barcelona will still control the game but they will lack the last push to find the opposition's goal.