Warren Moon was an undrafted free agent who even brought along his head coach (Hugh Campbell). Kurt Warner was a UDFA. Derek Anderson was a sixth-rounder that became the starting QB both in Cleveland and Arizona.
If he continues to start, Keenum will have half a season under his belt and, if he continues his current pace, he'll finish with a 99.0 QB rating, higher this year than Schaub (77.8), higher than Stafford (92.0), Newton (91.8), Roethlisberger (91.3), Ryan (89.2), Cutler (88.4), Luck (87.0), Dalton (85.2), Brady (83.6), RG3 (83.6) and Kaepernick (81.8).
But, of course, Keenum was never drafted so he'll never play again in this league.
To be fair, ESPN.com has the Total Quarterback Rating (QBR) and rates him at 42.5 out of 100, a ranking still better than Ryan Tannehill, Eli Manning, Matt Schaub, Terrell Pryor, Chad Henne, Geno Smith and Josh Freeman.