It has been reported by the British tabloid that the Spanish club want to secure the services of the Serbia international striker on a loan deal.
Newcastle, though, according to the report, would rather sell the 22-year-old on a permanent basis and get a transfer fee.
Mitrovic has been on the books of Newcastle since the summer of 2015 when he joined from Belgian giants Anderlecht for a transfer fee reported by BBC Sport to be worth £13 million.
Much was expected of the 22-year-old striker when he arrived at St. James’ Park, but the youngster has failed to make a massive impact so far.
During the 2015-16 campaign, the Serbia international scored nine goals and provided four assists in 22 Premier League starts for Newcastle.
Mitrovic was able to make only 11 starts for the Magpies in the Championship last season, scoring four goals and providing six assists in the process.