Magath's men travel to Wales for a crucial encounter between the Premier League's bottom two sides, a defeat for either could leave them in danger of being cut adrift at the foot of the table.
Mitroglou, signed from Olympiacos in January when Rene Meulensteen was still in charge at Craven Cottage, has played just 29 minutes of football for Fulham, in the 1-1 draw at West Brom on February 22.
The striker played an hour for Greece in their 2-0 defeat at the hands of South Korea on Wednesday, but Magath says he must wait and see if the 25-year-old will be available to face Cardiff.
"He is not match fit," he said. "He played only 60 minutes and has hamstring problems.
"We have to wait. He comes back today (Thursday), our doctor has to see him and maybe if he is fit he will play.
"We need a player in our situation who is prepared for 90 minutes' relegation fight.
"The Premier League is the best league in the world, so it cannot be compared with the Greek League.
"Every player who comes from the Greek League needs a few weeks to adapt.
"You cannot expect such a lot from a player who is new in the Premier League. You have to give him time to adapt. If he's not fit now, he will be fit next week."
