The Nigerian international career has been on an upward trajectory since an ill-fated six month spell at Ibrox, on loan from Roma, during the 2018/19 season, making just four appearances under Steven Gerrard.
Across the past four and a half seasons he has scored 69 goals for Perugia, Partizan Belgrade and Almeria, helping the latter win promotion to La Liga.
In his first three games in the Spanish top-flight he has scored twice, including the winner against Sevilla.
Real Sociedad, who finished sixth last season to qualify for the Europa League, have completed negotiations over a bumper deal which will see him sign on Wednesday, according to Fabrizio Romano.
Sadiq will be most remembered for his role in a League Cup semi-final defeat to Aberdeen when he was presented with a golden opportunity through on goal but opted to go down in the box instead.
In an interview with the Sunday Post, he claimed he was due wages from Rangers, that he was fined £20,000 for liking a social media post and was made to feel an outcast and not allowed to use the first-team car park.
“I believe absolutely that I was not given a fair chance to show what I can do. They made me feel like a stranger,” he said.
“As long as I was a Rangers player, I behaved like a professional, giving everything I could of myself to the shirt.
“But now I don’t want to hear about them and what they are doing any more."