He completed the 90 minutes, but he has been recovering since. Yesterday, he spent time in the gym rather than joining his teammates on the training pitch. The forward reportedly has no muscle injury, only a muscle spasm. That means the risk if he does indeed play is reduced. That fact has made the club quite calm over the situation.
Still, this is Messi we are talking about, and any injury to him is a huge problem. The player then, must decide if he thinks the pain has gone away enough that he will play tomorrow.
The player reportedly wants to take part, as he knows the importance of el clásico and winning the Copa del Rey. However, he is said to be weighing his options, as the cup is a two-legged affair and, in any case, the league title and the UEFA Champions League are higher priority competitions.
The medical team will give the player their opinion, but in the end, it may depend on how Messi feels. MD says Messi will not play if he thinks he is risking his performance in other competitions in the long run.