Busquets was on the end of a poor challenge from Gonzalo Escalante and left the field on a stretcher after 10 minutes, with Barca later confirming the Spain international had suffered a sprained external lateral ligament in his right ankle on Sunday.
It left Rakitic to fulfil much of Busquets' usual organisational responsibility in the visitors' midfield – something he did to fine effect as substitute Denis Suarez got his first goal for Barcelona before the break and the more familiar protagonists Lionel Messi, Luis Suarez and Neymar got in on the act during the second half.
"Each player has to play his best when we have the chance," Rakitic told Movistar. "Even more when someone gets injured like tonight.
"We are like a great family and we fight for each other.
"I am prepared for everything [to play in Busquets spot]. I want to play always.
"That is why I make what I can and I just try to show the coach I am ready for what he needs. I am very happy."
Escalante's tackle was not the only example of robust play from Eibar and Rakitic expected a rough ride.
"Eibar did their usual game at home," he said. "This is a very complicated ground to come to. We waited and had enough patience
"It's a deserved win and we need to continue like this"