The wide receiver missed the start of the campaign while serving a substance-abuse suspension, but has recorded 582 receiving yards and three touchdowns since returning in week three.
The quarterback told the team's official website: "I see him maturing. A lot of things have happened to him in the past that have caused him to take a really hard look in the mirror and see the things that he can really become and not let things get him in trouble and prevent him from being the player that he can be.
"From that standpoint, he's more of a quiet guy. He's maturing and understanding. I didn't realise he was that young. He's 22 years old, and it's almost like he's really a little brother.
"I always tell him to study film of the great ones that are playing right now, Calvin Johnson, Brandon Marshall, and continue to learn from those guys what they do best on a week-in, week-out basis to put up things that they're doing. Josh has all the ability in the world to be what he wants to be."
The Browns face the Baltimore Ravens later today.