The 26-year-old Frenchman has barely featured this season for the Gunners, having made just seven appearances and only one start in the Premier League in 2017-18.
Coquelin’s lack of action has made a January exit appear likely, with the likes of West Ham and Crystal Palace reportedly keen on acquiring his services.
But the combative defensive midfielder is set to make the switch to Spain and will undergo a medical with the Liga outfit on Wednesday evening.
Goal previously reported the Liga side would prefer a loan agreement with a purchase option, though it is now understood that it is likely to be an immediate permanent transfer with Coquelin agreeing a four-and-a-half year deal.
Coquelin has spent a decade in north London having made 160 appearances for the Gunners as well as having spells out on loan at Freiburg, Lorient and Charlton Athletic.
He finally seemed to break through into the Arsenal line-up following his time with the Addicks, playing in at least 20 Premier League games each of the next three seasons and starting a minimum of 19 each campaign.
Coquelin’s departure could be the first of many for Arsenal during the January transfer window, with Alexis Sanchez, Theo Walcott and others looking for moves away from Emirates Stadium,