from Jaguars.com: 

*Marrone on second-year linebacker Myles Jack: “The one thing I’ve learned about Myles is he takes a great deal of pride in knowing what do and how to do it and really performing at a high level. … He does a great job of learning and reacting and I just think he gets better and better. I think that this is only the beginning for Myles. He’s getting better in each quarter of football that we’re playing. He’s playing his best football now and I expect him to keep becoming a better player as his years in the league increase.”

Encouraging, considering he's still one of the youngest players on the roster.
From what I'm seeing while he isn't having really flashy plays he's helping a lot in coverage and taking away our old TE problem, yesterday in the second half he obliterated the Texans tight end for a great pass break up and on the Seahawks game he was constantly helping covering the middle of the field (esp on one of the INTs). 
Probably the player on defense who has the biggest margin for growth, especially in the run game.

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