David Moyes saw his La Liga side beaten as Nadir Ciftci made an immediate impact for new employers Celtic.
Gary Mackay-Steven scored the only goal of the game as Celtic warmed up for their upcoming UEFA Champions League qualifier with a 1-0 victory over Real Sociedad in Paisley.
Ronny Deila saw his side come out on top against their Liga opponents as recent recruit Nadir Ciftci played a key role in the 69th-minute winner.
The forward, who only completed his move from Dundee United on Thursday, needed just 60 seconds to make an impact for his new employers, his return pass allowing Mackay-Steven to break the deadlock.
Sociedad boss David Moyes saw his side hold possession for long periods against a club he previously played for - but the visitors lacked a cutting edge.
Sergio Canales hit the post in the opening half and Craig Gordon twice had to be sharp in the Celtic goal, firstly denying Yuri Berchiche before then turning away an effort from Imanol Agirretxe.
Ciftci was denied late on by visiting goalkeeper Geronimo Rulli, leaving Celtic to settle for a slender 1-0 triumph.
They are next in action on July 15, hosting Icelandic team Stjarnan in the second qualifying round for Europe's premier club competition.comments