The Barcelona midfielder was key alongside Luka Modric as the first-time finalists came from behind at half-time to win, thanks to an extra-time goal from Mario Mandzukic.
"Last night I had a fever, almost 39 [degrees]," he said.
"I was lying in bed finding the strength to play and it was worth it. I would play a final without a leg if necessary."
Rakitic also added that hype surrounding the England team on social media did not go unnoticed by Croatia and helped inspire them to victory.
"They thought they were already in the final, with all the things uploaded on [social] networks," he said.
"They can keep doing their stuff and we will play on Sunday."
Croatia take on France in Sunday's World Cup final in a rematch of the 1998 semi, in which Les Bleus triumphed 2-1.