Whitney Mercilus was selected by Houston with the 26th pick of the first round in the 2012 NFL Draft He was generally considered a raw talent since he didn't have a lot of experience, starting just 15 of 37 games in his time at Illinois.
Before entering the NFL Draft, Mercilus had a breakout season as a junior in 2011, notching 16 sacks and 9 forced fumbles; best in the country for both categories. Mercilus finished his college career with 81 tackles (42 solo) with 18.0 sacks (for a loss of 119 yards), 28 tackles for loss (for a loss of 150 yards), 11 forced fumbles and two fumble recoveries. 
In his final year at Illinois, Mercilus was a unanimous first-team All-American, won the 2011 Ted Hendricks Award, won the 2011 Bill Willis Award, and was recognized as the CFPA Defensive Performer of the Year.
As a Houston Texan, Mercilus has only missed one game in three seasons due to an injury. His career stats include: 18 sacks, 25 quarterback hits, 1 pass defended, 122 tackles (38 assists), 12 tackles for a loss, 2 forced fumbles and 1 fumble recovered. 
The Texans could have triggered a fifth-year option on his rookie contract under the terms of the current CBA, but they haven't done it yet. This might be a sign that they are prepared to move on from him after just four years unless they see something significant in his 2015 performance. Mercilus' peer, outside linebacker Brooks Reed, entered the NFL under the previous CBA and was not eligible for a fifth-year option; the Texans chose to allow Reed to depart in free agency to the Atlanta Falcons.
Mercilus has garnered positive comments for O'Brien, as Wikipedia includes this:
Mercilus moved into fifth place on the franchise’s all-time sacks list. He surpassed Kailee Wong, who had 15.0 in four seasons (from 2002-2006), and achieved 17.0 career sacks in just three seasons. "I really can't say enough about Whitney Mercilus," head coach Bill O'Brien said after the game. "I mean, he practices every snap in practice. He takes some scout team reps because we were a little bit thin at linebacker. Then, he goes in the game and he's played better and better every game. He's a great guy, a really good teammate, a hardworking guy. He's just played really well for us this year and I think that was what I saw out there from the sideline."
Ted Johnson of Sports Radio 610 has mentioned several times that Romeo Crennel's ('RAC") defensive system is very complex, especially for linebackers. Johnson played linebacker under Crennel for the New England Patriots when they won 4 AFC Championships and 3 Super Bowls. From first-hand experience, Johnson points out that once the players are more familiar with Crennel's scheme, they can become comfortable enough to play more instinctively and wreak havoc all over the field with the well disguised and disruptive nature of R.A.C.'s system. Getting to that comfort point just takes more time than in other defensive schemes. Johnson pointed out that the Texans' defense really started to look consistent and very effective toward the later part of the 2014 season. Ted said he is confident that they will pick up right where they left off heading into 2015.
2014 Mercilus Review
Matt Weston of Battle Red Blog begins revisiting the Houston Texans' 2013 season by looking at Whitney Mercilus's play last season.
2014 Mercilus Review
UPGRADES TO DEFENSIVE TALENT:
The Texans acquired defensive tackle Vince Wilfork to anchor the middle of the defensive line, and they signed free safety Rahim Moore to play a key "centerfielder" role in the secondary. Brian Cushing is feeling better this off-season than he has in years and also showed signs of being highly effective in this defensive system toward the end of last season. J.J. Watt will continue to move around and collect extra attention, including double-teams, which should provide opportunities for other players in one-on-one match-ups. With influence from these players dominating the middle of the field and interior line, outside linebackers should benefit in their pass-rushing role from the edge. If Jadeveon Clowney really is ahead of schedule on his recovery from surgery, then he should draw a lot of attention along the defensive front, which should further benefit Mercilus with extra opportunities.
When you consider all of these factors: contract year, fourth year starting in the NFL, second year in Crennel's defensive system, significant talent upgrades to the surrounding defensive roster, and competition from a healthy Jadeveon Clowney (or perhaps a rookie pass-rusher yet to be drafted), there is really no better opportunity for Whitney Mercilus to have a record season in Houston. It's time for Mercilus to take full advantage of the situation.
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