The Danish midfielder has struggled to make an impression at loan side Everton having only started three ties since he moved to Goodison in August.
Since joining the SPL champions from Real Madrid two years ago, the 32-year-old has failed to justify his £40,000-a-week wage.
His agent, John Sivebaek told the Daily Record: "It hasn't worked out for him yet at Celtic but he still dreams of making it there."