Johnson believes that Miller is relishing the opportunity to play regular football.
"I think it will be down to Ishmael really," he told Sky Sports News.
"He has come here to get his football going and I think he's enjoyed the two games and his one goal. That is what footballers want to do, they want to play and enjoy their football.
"But what they also realise is if you've missed a lot of games it is very hard to get your game going and your game fitness."
Miller joined Forest from West Bromwich Albion in 2011.