The reason undrafted matters in this case is because it means he was never being prepared for anything close to a starting role all year.  Dak Prescott was.  He was being groomed from the moment Dallas drafted him as the the immediate backup to Romo and future starter.  Even moreso for Winston, taken #1 in the entire draft.  And with Watson who was drafted #12 with the obvious intend to start grooming immediately for the starting job.

All those guys were being counted on to play either immediately, or soon.  Thus had 5 months of cramming, planning, and scheming behind their play.

Sloter wasn't (and still isn't) even expected to make the active roster.  He hasn't had anywhere near the grooming with the first and second teams that Prescott, Winston or Watson got.  

Trevor may not be the long-term answer, but he's pretty clearly the best choice of what we have for starting this season. The OL was horrible last year, the running game sucked, and the D had just won a SB by...