I understand what you are saying and is a valid point however I think you are minimizing the physical beating that QB's must take. As you have admitted Culpepper has been a pretty mobile QB so far in his career, relying on his legs to get him out of trouble or to create opportunities out of nothing. The injury not only might hinder his ability to move, but could also make him more suseptible to hits once he leaves the pocket. When a QB runs he becomes a RB, who naturally take a beating, but when a defensive player has a chance to get a clean hit on a QB his eyes come alive and he puts a little extra oomph into it. Many QB's get hurt when scrambling that are completely healthy, so there is even more risk for one that recently suffered a devastating knee injury.
You also don't mention that on every play the QB has several 250-320 lb. monsters trying like hell to take the QB's head off. Even is he doesn't have the ball, a QB is still a target for a DL or LB to try to get a quick hit in. The QB also generally stands near the line of scrimmage, were 8 or 9 300 lb. men are often rolling on the ground in a heap with no sense of up or down. Accidents happen, were an unsuspecting QB can get caught up in the line junk like Carson Palmer against Pittsburgh in the playoffs, or one of their own linemen could accidentally role over their leg. Linemen are not the most nimble players and sometimes can't get out of their own way. This threat of course is magnified by the fact that Culpepper may take a hit that a healthy QB would shrug off but because of the weaker knee he is knocked out of the game. Not to mention that a QB relies stable, well-planted looks for power and accuracy.
I also want to make sure you realize that QB's and DB's are just different types of athletes. They have different god given body types, but also train and build their muscles differently depending on the needs of their position. DB's are made to take more contact than a QB. It is possible that Rodney's different physical build could make him able to withstand more than Culpepper after the injury. At the same time Culpepper has an advantage over the average QB because he is quite large.
In an attempt to find a more suitable position comparison to a DB, you bring up RB's because you think they're position has similar physical demands as DB. I disagree because RB's have the shortest average careers of any other position and take the biggest beating. I think a more accurate comparison would be to a LB, especially in Rodney's case because he is a such a physical safety, who often plays like a linebacker. Many LB's and DB's play until the age of 35 or 36, while most RB's are pretty far gone by the age of 33. It is for these reasons I don't really think you can compare Rodney's injury to McGahee's. Also I don't know if you saw his injury but it was far uglier than Rodney's. His knee bent backwards where as Rodney's was more to the side. McGahee's injury resembled that to Theisman.
Rodney has the advantage of having his knee injury occurr about a month before Culpepper's, giving him more time to heal. Rodney has also said that he has always been a quick healer. Reports have been that both Rodney and Culpepper are doing well, so they might both be back for the season. I'm just can't bet against Rodney because he has overcome the odds his entire career and is one of the toughest and most driven players in the NFL.
