The 20-year-old midfielder, a former team-mate of Johnson, will remain at Boundary Park until the end of the season.
And Johnson believes his side have signed a quality player.
He told the club's official website: "He is unlucky not to be in Bristol City's first team. Had it not been for their league position, I am sure he would be because he is certainly ready.
"I have seen him progress from being a boy to become a man and a talented young player. He is a good lad, hungry which is important and keen to help get us out of trouble.
"It took him, as it did me in my career, to be around the age of 21 before I could compete with men.
"I think Bristol City kept the reins on him because of that, but now he is ready to explode.
"They are the sort of players we need to find, those who are under the radar."