Johnstone, 18, will go straight into the Iron squad for Saturday's League One game against Sheffield United.
The loan is for an intial month but is likely to be extended as Lillis is not expected to return from a knee injury until December.
"I think it's what I need. The experience at quite a young age will be good for me," he told BBC Radio Humberside.
"Hopefully we'll do well and I'll get some games under my belt. I went to see Sir Alex Ferguson and he told me to concentrate, do well and just enjoy it."