Arsenal are believed to be around £4m short of Real Madrid's valuation of loanee Dani Ceballos if they want to make the deal permanent in January or next summer.
The Gunners were able to snap up the Spain international ahead of Tottenham in July as he sought more regular first-team football after struggling to nail down a regular place at the Bernabeu.
A positive start to his career in north London has already led to calls for the north London club to try to sign Ceballos long-term, but it may require more negotiating in order to strike a deal.