There's a strong precedent in the history of Gibbs' running game to indicate that Portis will be spelled pretty frequently. Shane posted a great two-part article from KFFL that provides strong evidence to that effect:
http://thewarpath.net/showthread.php?t=475
http://thewarpath.net/showthread.php?t=476
That article also points out that Gibbs may have never had a back with Portis' prodigious talents (though many would call that sacrilege, given Riggins' Hall of Fame credentials). But even John Riggins shared a lot of time with Joe Washington. George Rogers shared a lot of carries with Riggins, and later with Kelvin Bryant. Earnest Byner's load was lightened considerably by the combination of Gerald Riggs and Ricky Ervins.
Given the current selection of runningbacks Gibbs has to choose from, I'd say he'll find at least two backs from that group that will allow him to keep Portis fresh-- as one poster put it-- "for the playoff run".
At this point, the Redskins are said to have around $4 million in cap room, and could enter training camp without needing to restructure or cut any salaries-- which could mean that Trung Canidate is safe. But if one or two other horses from this stable are able to get a fast start out of the gates, Canidate's salary would make him the first candidate for the waiver wire.
I agree with Shane, and I believe the darkhorse here is McCullough. He's got the raw talent, and if the coaches can harness it, he could be a factor down the homestretch. He and Canidate are similar in size and speed to Portis, but McCullough's younger and cheaper, so if Sultan struts his stuff in camp, look for him to become the primary backup at some point.
I loved Cartwright's tackle-busting tenacity last season, and Morton showed Bryant-like flashes of 3rd-down lightning. Betts remains mostly a mystery, but I agree that he's built like a prototypical Gibbs back. I hope he has a great camp and emerges as the Davis-like power runner we all envisioned he would be. The Skins would be downright scary with Portis being spelled by a hammering Betts to wear down the defenses.
Mike Sellers could be a monster as an H-back lead blocker. In his first incarnation as a Redskin, he showed a lot of ability in that regard. Let's hope the coaches can retrieve and refine that talent, and keep him out of the legal trouble that threatens to wreck his career. If Sellers and Betts step up, the Heavy Jumbo formation would be ready for action.
One thing's for sure, there are a lot of questions at the backup runningback position this offseason. The coaches get their first look at possible answers when minicamp opens March 26th. I can't wait...