Magath became Fulham's third manager in the space of a year after replacing Rene Meulensteen in February, though he was unable to avoid relegation from the Premier League in 2013-14.
Life in the Championship has been no better for Fulham, who sit bottom of the table having lost all four games thus far.
Saturday's 5-1 drubbing at Derby County increased the pressure on Magath, who has only won three games since arriving in London.
And former Fulham full-back Riise - released at the end of the last season – went on Norwegian television to voice his concerns with Magath, who is notorious for his brutal training regimes.
"With his arrival, they've flushed everything [that club stands for] down the toilet. His methods are ancient," said the 33-year-old.
"Spoke to a few players after the Derby match. Once they returned to London, Magath ordered a team meeting at 10.30pm.
"Then the next morning [normally day off/restitution], players were ordered to run for one hour, then another training session at 2.30pm. It's not the way to get the boys going. And training methods are terrible."
Riise added: "All football sessions are 11 v 11, working on tactics. Interrupted every five minutes. It's a shame, but I fear for Fulham.
"Responsibility lies with Magath and the guys upstairs [directors]. They knew exactly what they were getting with this guy.
"I really like Fulham. It's a real family club with a great stadium, but with Magath everything gets flushed down the toilet."
Get the best features, fun and footballing frolics straight to your inbox every week.
Thank you for signing up to Four Four Two. You will receive a verification email shortly.
There was a problem. Please refresh the page and try again.