Jim Schwartz has described playing cornerback in the NFL as the "most difficult thing" in sports.
The Detroit Lions have a number of young corners on their roster ahead of the 2013 season in a bid to strengthen their defensive play.
The team's head coach told the Lions' official website: "It's tough to play corner. I think it's probably the most difficult thing to do in all of sports.
"You have to run with world-class receivers, you've got to start backwards and they're going forward. They know where they're running, you don't. You've got to be tough enough to take on pulling guards and running backs and skilled enough and fast enough to cover the elite athletes, you know, guys that are Olympic-caliber speed.
"I think that's been more of our emphasis, trying to find guys that have all of it. They are rare to find, something you have to develop, which we're working on with Chris Greenwood. Other guys, like [Ron] Bartell is a veteran player, we had the opportunity to get and we were able to plug them in."
The Lions used their second-round pick in this year's draft to acquire cornerback Darius Slay.