The Copa del Rey was the least important competition available to FC Barcelona until they were drawn against Real Madrid. This changes everything and raises the bar. Ernesto Valverde is now forced to field his best available team. The Camp Nou will be full, Messi will be forced to play and the reappearance of Dembele, also injured, is also needed. Now this competition means everything. We forget the words the captain said at the start of the season. Plans change, priorities change.
Barça, and Valverde in particular, must be more ruthless. The priority is LaLiga due to how consistent the teams needs to be and Champions League is what excites everyone the most. Barça have won seven of the last ten LaLiga (25 in total), six of the last ten Copa del Reys (or four of the last four, which makes it 30 in total) and three of the last ten Champions Leagues (5 in total). The Copa was the competition that "if we win, good, and if not, other year". That can't be allowed to change now.
Valverde has rotated his side in recent weeks to arrive fresh in the crucial part of the season. He must now ensure that this epic contest doesn't hurt Barça's long term plans this season. If he believes rotation is key, he can't change that even if the rival is Real Madrid and there's a desire to halt Solari's good run of form.
Barça must continue along the same path, regardless of who the rival is in the Copa, so that the Copa doesn't make Barça slip up in the Champions League. It doesn't matter if people think the team will go through, they said the same against Roma and Juventus, it's important to see the job through. That's why Barça must be careful with the Copa.