Johnstone is delighted with his opportunity with the Saddlers.
"I work every day with David De Gea, Anders Lindegaard and Ben Amos," he told the Express & Star. "I am working with some of the best goalkeepers in the world. It's great experience and we are all young goalkeepers. That's all great, but I need the experience of League games so I'm grateful for this chance.
"I'm just glad Walsall came in for me and I'm excited to come and play here. I was at Scunthorpe for five months last season and that was a great experience that helped me to grow up. It's proper football in a proper league - with crowds, where you're playing for points. It's the best thing for me. I wanted it and United wanted it.
"I couldn't go out at the start of the season because Ben was at Hull but, once he came back to United, I wanted to take my chance to go out and learn, and to pick up experience, which you need as a goalkeeper. Hopefully I'm coming in here to help them get into the play-offs. I'm used to winning games at United so I want more of the same."