He fell out of favour at Anfield last season after claiming that Liverpool was a "sinking ship" under former manager Rafael Benitez.
Riera was greeted by thousands of Olympiakos fans on Friday and admitted he was taken aback by the reception.
"I was told the fans were coming but what I saw was much more than I expected," he stated. "I did not expect so many people, honestly. It was a surprise for me.
"Such enthusiasm is good for the club. I understand how important the support of the people is, it's priceless."
Prior to joining Liverpool from Espanyol, Riera played for Bordeaux and Manchester City.
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