Riordan appears to have become tired of sitting on the bench

Celtic forward Derek Riordan could face showdown talks with manager Gordon Strachan over his future.

The out-of-favour 25-year-old has returned to training after being on compassionate leave following the death of a relative, BBC Sport believes.

But he was quoted in a weekend newspaper criticising Strachan.

And it was then disclosed that he had asked the players' union to assist him in challenging a �1000 fine imposed by the Glasgow club.

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Neither side has been willing to comment on the matter.

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But it is the latest sign of the player's unrest at Celtic Park.

Riordan has made just 11 starts since arriving from Hibernian in summer 2006.

And his 11 outings this season have mainly come from the bench, his last being a six-minute cameo appearance as Aberdeen knocked Celtic out of the Scottish Cup on 18 March.

Riordan, who has still managed eight goals for the club, had hoped to leave Celtic in January, but a cash bid from Burnley and a loan offer from Hibs were rejected.