The Greek forward is now back in training and expected to push for a first-team place in the second part of the season.
Despite the arrival of Japanese forward Daizen Maeda last month, Postecoglou is adamant that the 27-year-old, six-times capped target man will succeed at the club. He scored 26 goals in the Dutch top flight last season and has been backed to hit those heights again.
"I believe in Giorgos,” Postecoglou said in an interview with a Greek TV station. “He had a great season with Venlo last year.
"He came after pre-season and he was a bit unlucky.
"Celtic is a great opportunity for his career. You saw what happened with Georgios Samaras."