The Chilean international has been linked with a summer move to Napoli, but Bartomeu is determined to keep him in Spain.
"He's not for sale," he told El Mercurio. "We are confident in his ability.
"It is speculated so much that he might leave, but there is nothing - no formal offers have arrived for the player and he will remain here.
"We are pleased with his performances and we will not put him on the market."
The 24-year-old joined the Spanish champions from Udinese two years ago in a deal worth £22.7m.