City's 3-2 win on Tuesday night rounded off a 5-3 aggregate victory.
But despite the loss, Johnson was sure his players would take away confidence from challenging one of best teams he has seen play live.
"We came close to a draw today. They are a top, top side, probably the best side I've seen live," said Johnson.
"They have so many players that are intricate and have class on the ball and sometimes you have to hold your hands up and say 'they are a better side than us'.
"But the spirit we showed, we never gave in and we continued to try and attack and scored two goals and not many teams can score two goals against a side like that.
"It's a good lesson for the boys and I think we'll take that into the Championship. It's a positive lesson and we're a good side in the Championship and now we'll get to focus on the league and it will be a really good run in until the end of the season."