Johnson had a hand in Sergio Aguero's late winner.
Johnson told Sky Sports 2: "It was a tough game, it's been a long week and a lot of travelling, but we've come away with the win and we're through to the semi-final.
"We changed the team around, it was the second-stringers if you like, but we're progressing as a team and as a club and I hope it continues."
That progress has seen Johnson play only a fringe role for much of this season, but he said: "For me and the lads who are playing in this competition, it's a chance to try and get in the team week in, week out.
"And if we can get into the final and win another trophy, it's important for the club."