Kevin O'Connell
Kidding aside, Guyton has to be it. No one else played enough (or consistently enough) to be considered.
Wheatley was getting better but got hurt, so we don't know if he would continue to trend upwards or fall back to earth.
Wilhite also improved as the year went on, but he went from being a liability in coverage to holding his own. About the best you could expect from a 4th round corner.
Slater was good at some aspects of special teams, but made some mistakes too.
I would give an honorable mention to Law Firm. He saved our bacon for a couple games by giving us a solid running option next to Faulk.