Shanahan also revealed that starting cornerback Ahkello Witherspoon will miss "at least a month" with a foot sprain.
The good news is that Coleman and Hurd are expected back soon. In fact, Shanahan said there is a good chance that the two will return in time for the 49ers' Week 5 matchup against the Cleveland Browns on Monday, October 7.
"I think [Coleman has] got a good chance to come back next week," Shanahan told reporters. "He was just jogging and moving pretty well last week, just watching him run on the side. He won't be out there tomorrow (the 49ers' last practice before breaking for the bye week) but, hopefully, we'll get him practicing here next Monday."
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Shanahan didn't want to speculate what might happen to the current group of running backs, made up of Matt Breida, Raheem Mostert, and Jeff Wilson, Jr.
"It's pretty tough to use four backs throughout a game," Shanahan said. "But that's a problem I'm looking forward to having, to try to figure out."
As for Hurd...
"This will be a big week for him with rehab and stuff while we're gone," Shanahan said. "But yeah, I'm hoping he'll be back (at practice) next Monday, and I'm expecting him to."
Shanahan noted that Hurd's injury was serious, and that's why he had to rest his back for about a month.
"We're getting close here to where he should be ready to go, so hopefully, it will work out for him," Shanahan added. "You never know when they come back out if that can act up again, but we feel it's healing up, and hopefully he'll be able to come out here and help us."