Interesting question regarding trading a RB. Every single one is filling a role that isn't the same as any other. While the question is a lot easier than analyzing each player (cap impact probably makes certain players not tradable):
Gill is statistically the surest 3 yards on the team -- short yardage needed for first down. Decent power w fair moves, he is the wear them down runner. Adds extremely little to the pass game. Durability good so far. Unless Burkhead took this role, unlikely, Gill is indispensable.
White, is the third down back, the main passing threat, and he can take some-ish runs and be productive. He's our Vereen, Whitehead, Faulk position and is doing it well. So far he has shown to be durable. Unless Lewis was to take this role, White is indispensable.
Lewis is, IMHO, clearly the biggest threat to break tackles. This season he's been the between White and Gill -- getting action in the run game while catching some/a bit of passes. Durability concern even before the Patriots. But a healthy Lewis is an asset with clearly the best moves of the group. Keep his snapcount right -- health -- and he is a dame fine 'tween' for Gill and White. Also now contributing a key spot on ST. If White gets hurt he is the backup.
Burkhead is the wildcard. The biggest of the 4 (5'10" 215). Durability isn't quite yet a concern but he was hurt. Since returning he has shown good production both run and pass. I assume(?) he is the backup if Gill gets hurt. Could he be the third down back in a pinch??
Burkhead and Lewis, as different as their 'moves' and size are, actually overlap the most? Huh.
Unless the player to be named in the trade is going to make a tangible impact in a position we have to have, I'd hate to lose any one of these guys. As the weather gets colder and potentially messier, being able to give such differing looks/skills? Being able to keep each one's snap count down/fresher? When one isn't working on any given Sunday being able to try the next or the next to see if he can breakout? With injuries always on the horizon? Given our passing game may not be as potent as last year making the run game even more critical?
Lots of reasons to keep all 4 but RB seems to be where our cup most runnith over if trade bait is needed. Unfortunately I think it will be Lewis who is traded (again cap implications as well as contract status that make a trade viable/not viable is not considered here).
