This is the biggest mark against him. It’s not “one game.” He’s been a below-average DC his entire time in Detroit.
This year, the injuries hit hard, but that doesn’t explain the other years. Dennis Allen, for all his flaws as a head coach, was much better at scheming around injuries and lack of talent than Glenn has been.
So unless Glenn is truly a magical personality whose mere presence can motivate a team to new heights, I don’t see why he should be near the top of the wish list.