The trio all impressed at the tournament, with Ruiz in particular catching the eye for Costa Rica with a match-winning performance against Italy.
Magath told Get West London: "The players need a rest.
"They have three weeks holiday (after the World Cup). Dejagah and Mitroglou are due back on Sunday and Ruiz is coming back next week.
"I am sure they will still be with us in September."
The Championship season gets underway on August 9 and Fulham will be looking to bounce back to the Premier League at the first attempt following their relegation in the last campaign.