"The Mauler" was set to headline the show, which takes place in his native country of Sweden, against Strikeforce, Dream and Pride veteran Gegard Mousasi.
The initial news came from Swedish media outlet Sportspegeln, but few details are available other than the fact that it does not appear that Gustafsson will be able to compete Saturday. Should the report be accurate, the timing couldn't be much worse for Gustafsson, the victor in his past six fights, who was promised a title shot with a win over "The Dream Catcher."
The lanky striker last competed at UFC on Fox 5 in December, earning a convincing unanimous decision over Mauricio "Shogun" Rua.
Meanwhile, Mousasi is on a 5-0-1 run in his past six bouts, most recently finishing Mike Kyle with a rear-naked choke at the Strikeforce finale in January.
Mousasi boasts one of the best records in professional MMA with a mark of 33-3-2, though it's been years since he's been tested by truly elite-level competition at 205 pounds.
Gustafsson, currently ranked No. 2 on the UFC's official light heavyweight rankings, certainly would have provided that challenge.
No word if there is any hope of another light heavyweight stepping in to face Mousasi, ranked No. 10 in the UFC's rankings, on very short notice.
The co-headlining bout for UFC on Fuel 9 is currently a lightweight scrap between Ross Pearson and Ryan Couture, so there is always the possibility that matchup could be promoted to the main event if no one steps up to face Mousasi.
MMA Fighting's Ariel Helwani spoke to UFC President Dana White, who is stating that Gustafsson vs. Mousasi is still on.