The 18-year-old was handed his first start for the club in the 1-0 win over Yeovil and kept his place for the 2-1 victory against Crawley.
"Glenn has done very well and, as many know, he has not had a lot of coaching in the game as he came late into football," Johnson told the club's official website.
"What he has are the raw attributes. He is quick, strong and aggressive and is a very good one-on-one defender, probably the best we have at the club.
"He needs to learn positioning, when and how to pass the ball properly. What we have is a rough diamond and, if we can spend a bit of time on him, then he can be polished."
- Sports & Recreation