If Sims is healthy, he'll be given every opportunity to have a role. (likely as passing down back this year)
He was already ahead of Rainey as passing down back before injury. Rainey's already lost 3 fumbles in 41 carries. So bypassing him won't be an issue. And Martin's been lackluster since the beginning of 2013. I've never been a huge Doug Martin guy but I think he's a better than what the stats show & should remain the guy (unless he keeps averaging 2.1 ypc

which I don't see )
Sims is a rookie who broke his ankle about 8 weeks ago. The timetable on that injury is 12-14 weeks. He might be around for the last few weeks of the season to get some "see what you can do" work.
http://thepewterplank.com/2014/10/10/charles-sims-strides-towards-early-return/
Charles Sims Is Making Positive Strides Towards Early Return
In August, the Tampa Bay Buccaneers were dealt a punishing blow to their backfield when their 2014 third-round pick Charles Sims broke his ankle during a team practice in the preseason. Initial reports indicated that Sims would stand to miss anywhere from 12-14 weeks after undergoing ankle surgery. At best, the Buccaneers were hoping to have their running back out of West Virginia anywhere from the second week of November to the end of the month, but it looks like he may be coming back sooner.
As spotted by
Jenna Laine, Sims is already getting in some work on the field and cutting, which supports the claim made by Bucs general manager Jason Licht, who said that he was “way ahead of schedule” and could make his debut when the team takes on the Cleveland Browns on November 2.
Given the struggles of Doug Martin, who is averaging 2.5 yards per carry this season, Sims could make a case for the starting running back job in his debut game. After all, this is the same guy who
received a comparison to the great Matt Forte by his own head coach. If that doesn’t get you excited about what this kid can potentially offer when he gets out on the field, we don’t know what will.