Two things will govern Marc Colombo's fate:
1. Tyron Smith beating him out in training camp
2. Sam Young winning the swing tackle role
Colombo has lost much of his lateral movement. He loses too many battles to capable, speedy pass rushers on Tony Romo's right side.
If Smith comes in as a rookie and demonstrates that he can hold up a pass rusher for even slightly longer than Colombo, he's getting knocked back to a backup role. He's not an option at center as starter or backup, so scratch that. He does not run well enough, so he cannot pull effectively on traps, counters or sweeps. Starting or backing up at guard makes him a liability as well.
Should Smith elevate to number one right tackle, Colombo is looking at swing tackle duty. Young is no longer a fresh rookie. This is where Hudson Houck's evaluation is key. If Houck believes Young is a solid swing tackle, that's it for Colombo. He's not going to fall onto the practice squad.
This offensive line needs Colombo's spirit, but he's facing an uphill climb which his knees fail him if Smith and Young prove they are both capable right away. I feel for him in his situation. He's kinda like Achilles with an arrow in his heel, but there are two arrows in both knees in his case. His warrior's body betrays him.