The Greek striker is lost potentially for an extended period before he has even played a minute for the club he joined at the close of window in a £2.5million deal from Dutch club VVV-Venlo.
Expected to be given his first game time in what proved a 3-0 Premier Sports Cup quarter-final victory that has set-up a last four encounter with holder St Johnstone in late November, Giakoumakis pulled up with a calf problem in the warm-up, forcing him to be sent for a scan.
It left Postecoglou despairing as he has had to cope without Callum McGregor, Kyogo Furuhashi, James Forrest and Greg Taylor after all were sidelined in recent weeks.
“I don’t know what it is mate, I have definitely walked under a ladder,” Postecoglou said of his accursed luck with injuries among his Celtic squad of late. “He did his calf in the warm-up.
"We’ve sent him off for a scan. So we’ll just wait and see. There’s something looming over me at the moment, just to challenge me every day. We’ll just have to cope with that.
"I haven’t spoken to him and I only found out about it at half-time when I was looking to make some changes.
“He’ll be disappointed and frustrated because he’s really keen to get his career going
“But we didn’t sign him for two months. We signed him for a significant amount of time and we want him to have a good career here.
“It’s just one of those things. It’s been really unsettled, having players coming in when we have games on.
‘We’ve tried to be careful with him and a few others but such is our predicament at the moment.
“He’s just got to be positive.”