The Tampa Bay Buccaneers could have kept running back Doug Martin in town through the 2016 season, by picking up the standard fifth-year option embedded in first-round draft picks' contracts. As Rick Stroud points out, the Buccaneers declined to pick up that option. We know, because the deadline was yesterday.
Had the Bucs picked up his option, they wouldn't have been committed to paying him anything in 2016: the money would have been guaranteed for injury only, so they'd only have risked him suffering a season-ending injury that would have carried over well into next season. That's not a completely insignificant risk, especially with the option likely being worth around $3.5 million, but it shouldn't be a massive problem had they actually liked Martin even a little bit.
A few weeks ago, Lovie Smith said that they "hadn't seen the real Doug Martin yet." Apparently, they now won't. It seems more likely that the Bucs will give him some touches this year, but will focus on players who they're likely to actually keep instead -- players like Charles Sims and Bobby Rainey.
The fact that the Bucs didn't draft a running back this year is a little curious if they wanted to move on from the Muscle Hamster, though, but other needs logically took priority. The Bucs do actually have a decent stable of backs, just no stand-out runners.