The Spain striker was back with his team-mates on Monday after twice being hospitalised last week with kidney stones.
But he has been left out of the squad for Wednesday's clash in Milan and will remain in Barcelona to work on his fitness.
"Today I returned to train with my team mates, but after the colic of last week I am still not 100%..." he wrote on his Twitter account.
".. So we have decided to stay in Barcelona and train hard these two days! Thank you to everyone!"