Hangeland clashed heads with team-mate Kieran Richardson midway through the first half at Craven Cottage, and Magath is convinced they would not have lost 3-1 had he remained on the pitch.
Fulham travel to relegation rivals Cardiff City next weekend with both sides desperate for three points to avoid being cut adrift at the bottom to the table.
Magath remains hopeful that the 32-year-old will make a swift return to training so he can lead the side in one of their most important games of the campaign.
"It was a big problem our captain was injured and wasn't there in the second half," the German said. "I don't know how Brede is.
"I hope he will come back and join us next week. I don't think we would have made such mistakes with Brede.
“He is the heart of the team, and it was under control with him.
"He is a very important player for us. He is going to hospital so we have to wait.
"We hope he will be back next week to stabilise us."
Magath was critical of Fulham's defending in the defeat on Saturday and vowed to put his player's through more training drills to ensure errors were not repeated.
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.