His eight league starts in Glasgow have netted five goals, but Riordan was more productive in Edinburgh, where he turned professional with Hibs and scored 54 goals in 123 SPL appearances.
He produced 23 goals during the 2004-05 season and 20 over the course of the following campaign, earning himself a transfer to Glasgow.
Despite his form with the Old Firm giants, they are pricing Riordan in the £1m range.