Having defeated Manchester United in the last eight, Johnson takes his team to the Etihad Stadium on Tuesday night bidding to cause another surprise in the competition.
However, the Robins manager has acknowledged the size of the task awaiting his side, as well as claiming that the current City team is one of the best to ever grace English football.
The 36-year-old is quoted by PA as saying: "I think they are up there with the best that England has ever seen. There's been some great teams in the past - Sir Alex Ferguson's Man United with the 'Class of 92' and before that the great Liverpool sides.
"But I think as time goes more than likely this team will etch itself in history as one of the best teams that have ever graced the pitches of England.
"They've got world-class players in every position, they've got world-class players behind that, and they've got Under-23 world-class players behind that."
The Championship outfit head into the fixture having lost their last three matches in all competitions.