I would be trading for Tatum Bell in a re-draft league. I would be trading Tatum Bell away in a dynasty league.
Every year since Davis retired it seems Shanny has had a new back to insert. It was Gary, then Anderson, then Droughns, then Anderson again...now Bell. It's not just that you can't trust Shanny...is Bell really that good?
I believe Shanny will either draft someone next season or possibly sign someone that is in a back up role now, although he usually doesn't make those types his premier back for the year.
Shanny would be a fool to just keep Tatum as the starter going into next season. This is a guy that traded Portis away...so he really has a high bar for RB if they could ever reach that bar for Denver to pay enough $$$ to keep them around. They seem intent on signing guy out of college to 3-4 year deals and then letting them go before they get too expensive.
Dynasty League...trade him while he is worth a lot...maybe let him tear it up another week or two and then get another RB that has a much more settled situation.
First, regarding the Portis situation, we don't know anything for certain. Shanahan's motives could have been influenced by something that stayed behind closed doors for all we know. However, based on what we do know, I would draw a different conclusion that you did. Portis was incredible in Denver, no doubt. He also wanted incredible RB money. It's not that Shanahan has a "high bar" at RB in order to pay a player a lot of money. If you were going to pay any RB big money at that time, Portis was in the elite group that deserved it. However, it doesn't make sense to blow all that money on a top rate RB when a minimum/rookie salary RB can achieve similar results. The cost-benefit ratio of a top-flight RB compared to that of a less talented, cheaper RB didn't make sense for the team. The Broncos had the opposite problem at CB. No matter what they did, it seemed that they couldn't get a decent CB out of the draft. The cost-benefit ratio at CB made sense for them. The result of that very logical trade is that, in two seasons, Denver's defense has has become the dominant aspect of the team. Keep in mind that this is a traditionally offensive team. Are they missing Portis? Sure, but it's pretty evident that this season's offensive problems aren't stemming from the RB position.