Gustafsson was expected to make fairly light work of Johnson on his way to booking a match-up with light heavyweight champion Jon Jones.
However, in front of 30,000 people at the Tele2 Arena in Stockholm, things went horribly wrong for the Swede who suffered a TKO defeat in the first round.
Speaking after the bout, Gustafsson, who broke down in tears following the defeat, admitted that losing in front of his home fans made the result even harder to come to terms with.
"It's just a lot of pain," he said.
"I not only lost my fight, I lost my fight in front of 30,000 fans, so that's a tough thing to deal with, and it still is. But whatever hurts you gets you stronger."
The 28-year-old conceded that he was beaten by a fighter who was better able to use the pressure of the occasion.
"My game plan was to work every round, tactically, with my distance and my speed," he explained.
"But Anthony's a beast and he came in with his pressure. He did what he had to do. And he did it good."