Not all mormons go on their "mission." On these missions, they go to a foreign land and try to spread their religion or something like that. It isn't like the Hajj in the Muslim religion, which all Muslims are supposed to do once in their lifetime. Denney has great speed for a DE and could actually be a problem. There are quite a few mormons in the NFL and this hasn't really been a problem from what I know, and Steve Young was a mormon but he is gone.
I know all about Mormon missions - so here are the actual facts from an actual Mormon. The missions are paid for by the missionary and the individual is sent anywhere in the world - from his own hometown to the farthest outreaches imaginable. Missions are elective and men go for 2 years and women for 18 mos. Typically, guys leave on their missions when they are 19, and that is why many Momron guys just leave on their mission immediately after high school and start college after the mission, rather than having a break in their college career. You pay your own way (approx. $375/month before spending money) and live very humbly with a companion preaching and volunteering and serving in another community. So that's what he did on his mission. Hope that helps! For more info, go to http://www.lds.org/newsroom/page/0,15606,4037-1---6-168,00.html