#2 Not experimenting with a new formation
Real have played with a 4-3-3 formation in almost all the matches this season, and Solari has not experimented with a new one in spite of not getting the desired results. Their defensive frailties were exposed by a top attacking team like Barcelona, and yet Solari did not try the 4-2-3-1 formation for a change.
Playing Casemiro and Federico Valverde as a double pivot would have strengthened their defence somewhat, as it would ensure better cover for their centre-backs. Casemiro is a good tackler and marker, and having Valverde by his side would have allowed him to go for early tackles against the likes of Lionel Messi and Luis Suarez.
It is quite clear that Toni Kroos does not contribute enough defensively, and therefore, he could have been left out of the playing eleven against Barcelona. Instead, Solari went ahead with the midfield combination of Kroos and Luka Modric, and paid the price. He took off Kroos and introduced Valverde in the second half, but the damage was already done by then.
Playing Valverde and Casemiro as defensive midfielders would also have allowed Modric to go into attacks frequently and also play timely passes to the wingers Vinicius and Lucas Vazquez. Kroos is a very good passer, but does not quite retain possession as much as he should while playing in a 4-3-3 formation.