The Latics boss described the 18-year-old as a "rough diamond" and vowed to help him improve his game.
"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 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."
Belezika recently broke into the Oldham first team, starting in the 1-0 win over Yeovil Town and keeping his place for the 2-1 defeat of Crawley Town.