Let me start by saying I like Broussard, his speed is real (unlike Matt Jones'), he can catch, and he's competitive. The Jags had to keep him on the roster last year because his quickness wouldn't have been overlooked and he'd have been picked up off waivers. However, that most likely won't be the case this year. He showed his lack of size was an issue last season by going out and getting injured late in the year and finishing on IR. According to the Jags' coaches, he was easily rerouted on press coverage and hasn't learned enough techniques to make up for his lack of size in the NFL.
John Broussard can't block... Period... He has been able to mitigate this problem's affect on his play time with his deep speed. His speed has forced one more defensive player to stay out of the "box" and account for him deep. In a run-first team, if you can't do something to help the run game, you're toast, and luckily for him, his speed made up for his inability to block.
In Jacksonville, blocking is sacred, nay, it is downright holy. Contrarily, to not love blocking with every bit of your heart is near blasphemy in Duval County!
Del Rio wants physical play on both sides of the ball, that's his mentality and it's been instilled in the players. Broussard's physically unable to do that. He can't go over the middle, he can't block, and his tiny frame makes him injury prone. He may clear waivers if cut this season and he'd end up on the practice squad.
There's a great article on Broussard I've linked to below.