The 6-foot-3, 290-pound former first-round draft pick sustained a fractured tibia in the first game of last season and spent the year on injured reserve.
Castillo visited the New England Patriots after being released, but ultimately re-signed with his original employer.
For his career, he has 276 tackles, 19 sacks and 43 tackles for loss in 81 games with 78 starts.
Castillo has also started seven playoff games with 20 tackles, two sacks and four tackles for loss.
-- Aaron Wilson, Scout.com
LSU Tigers Michael Brockers
(Mark J. Rebilas-US PRESSWIRE)
With Castillo off the table, it's still possible the Patriots will retain Gerard Warren, who has been with the Patriots the past two seasons on one-year deals. Warren, a former first round pick (3rd overall) of the Cleveland Browns in the 2001 NFL Draft.
New England needs to upgrade the defensive line to give Vince Wilfork some help inside. Often double, even triple teamed, Wilfork remains a constant presence for the Patriots interior. Castillo, an end who could play inside, would have alleviated some of the pressure on Wilfork to keep the defensive front stout up front.
The Patriots have Kyle Love, Ron Brace, Myron Pryor and recently re-signed Marcus Harrison as their current group inside. Brandon Deaderick is the only other true end with significant experience from last season. Depth was added recently via free agency with Jonathan Fanene who is expected to replace departed Mark Anderson.
Brace and Pryor dealt with injuries in 2011, Harrison was out of football. Love was the team's only real presence at the position aside from Warren and Wilfork. Deaderick played on the outside.
The Patriots will undoubtedly add depth to the roster via the Draft. The team appears high on LSU lineman Mike Brockers and Memphis Dontari Poe. Penn State defender Devon Still is another name on the radar, though the Patriots have typically steered clear of Penn State defenders and have never drafted one under Bill Belichick's tenure in New England.
Here are the Top prospects to keep an eye on: Scout NFL Draft DT Rankings
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