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Free agency makes the combine full of unknowns.
The information to be had at the NFL Scouting Combine is limited by those who provide it. There are 334 invitees to the combine this year, though not all of them will be drafted and not all of those who do land with a team in April will be at the event.
Those present may not even participate in all the drills, whether because of injury or because they'd prefer to do them at their own pro days, which means the Bengals won't be able to find out everything they need to know about every prospect on their big board.
Take Alabama running back Eddie Lacy, for example. Considered the best running back prospect in this year's draft and someone whom the Bengals may seriously consider taking with their first-round draft pick, Lacy won't be running at the combine as he's recovering from a hamstring injury. So any running will have to wait until Lacy's pro day, if he's healthy enough.
Though at this point we know what the Bengals' biggest positional needs are for 2013, we don't know how they'll fill them. The draft, of course, is a major source of young talent, but Cincinnati's strategy won't begin to take real shape until free agency begins.
Michael Johnson is about to be an unrestricted free agent, but how badly they need a defensive end will be based on their ability to keep him. Draft needs will shift drastically after the combine is over.
While we might know what positions the Bengals need to address, and while we might see certain players at those positions do well at the combine, that doesn't automatically mean the Bengals will draft them. There are factors—some in the Bengals control, others not at all—that inform draft-day decision-making, and a defensive back's 40-yard dash time is but one of them.