The striker, who had previously featured for just 30 minutes in a Fulham shirt, played for 90 minutes against Cardiff but struggled to make a real impact.
And Magath revealed Mitroglou, who fired in 21 goals for Olympiacos this season before his departure, was experiencing a culture shock.
"Mitroglou is a player who is not used to a relegation fight," the German said.
"He comes from the best club in Greece, who was better than every other in their league, so he is only used to scoring. That is his strength.
"Here in the Premier League, the best league in the world, he is not used to it. Unfortunately he has some opportunities but at the moment he is not in the situation to score."
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