Johnstone has had a spell on-loan at Scunthorpe United.
"It was good as it helped me a lot. It was a difficult time when I was there as they were fighting relegation and hovering around the drop-zone. It was a good team but they just couldn't get the results. It helped me to grow up from a little boy and into a man so it was all about maturing. You have to do everything for yourself and it was different," he told manutd.com.