City are four points clear of Nottingham Forest in the race for automatic promotion after both sides suffered poor results over Easter.
"I don't know if it is a case of nerves or what and I guess we won't know until the 46th game of the season, but we have got to keep going," said Johnson.
"But if we'd have said at the start of the season we would be four points clear of the play-offs with four games to go we'd have taken it.
"We have to hang on in there and be strong and certain players have to come out of their shells a bit."