One of the reasons injuries in the NFL are a little tricky is because you have some players that no matter what do not want to leave the field. Some guys are worried that if they step off the field because of an injury (newsworthy injury lately is concussions), they may not get their job back if the replacement is pretty good.
Jackson left the Week 1 game against San Diego with a knee injury and hasn't returned since. O'Callaghan hasn't played all year. They were both starters last year.
In their absence, the Chiefs have played pretty well. And they haven't been messing with any roster moves or any depth chart changes in that time. Most would say DE Shaun Smith and RT Barry Richardson have been good replacements.
So how will Jackson and O'Callaghan fit back in with the Chiefs? It'll be interesting to see how Todd Haley, Romeo Crennel and Charlie Weis work these guys back in.
On the one hand, those guys are starters for a reason and you want them out there. On the other hand, why fix what's not broken?