Marcel Jones is the backup RT. He can't play LT because he doesn't have the foot speed. Charles Brown has the ability to be a pro bowl LT, but he is inconsistent, unproven, and too injury prone to count on him yet.
And too inexperienced since he started college as a TE and was moved his JR year to OT. He showed improvement last year but still looked about a year away from starting. Hopefully he takes a major step this offseason.