Santos leaves PAOK Salonika
The Portuguese, 55, opted not to renew his contract after guiding the team into the Champions League qualifying rounds by winning the play-offs.
The Northern club would be bidding to reach the group stages of Europe's elite competition for the first time.
"I made my decision two months ago," he told a news conference on Tuesday.
"For me it was a great honour and pleasure to be able to help get this team on the right track, but the last three years have also been exhausting. You have to be able to give 24 hours a day, seven days a week and handle the pressure of matches and I don't feel that way right now."
The former Porto coach has also had spells at Panathinaikos and AEK Athens, whom he led to Greek Cup success in 2002.