It’s an inexact science, evaluating football players heading into free agency and the NFL draft. And the challenge is compounded for NFL teams that aren’t quite sure about the quality and depth of talent they already have under contract. That describes the Washington Redskins (7-8) as they wade into the pool of running backs expected to be available this offseason. Following an injury-hampered 2017 season in which nine running backs have cycled through the roster — none making more than seven starts — do the Redskins have their lockdown starter in second-year player Rob Kelley or rookie Samaje Perine? Could a preseason casualty such as Keith Marshall or a late-season signee such as Kapri Bibbs contend? Or do they need to upgrade? “There is something to like about all these guys,” Coach Jay Gruden said this week when asked about the team’s revolving running back corps, “but from a consistent production standpoint other than Chris [Thompson], we didn’t get a lot of probably what we are looking for from any of those guys. Not to say they did poorly — but it’s just hard to evaluate them and say, ‘Hey, we don’t need a running back because we have A, B, C, D, E.’ ” Gruden proceeded down the list. Marshall, a former Georgia back whom the Redskins drafted in the seventh round in 2016, remains on injured reserve after suffering a knee injury in training camp that derailed him for a second consecutive season. Just a few days earlier, Gruden had predicted great things for the 5-foot-11, 222-pound player. Kelley, who opened the season as the starter, rushed for 78 yards in the Week 2 victory over the Los Angeles Rams but spent subsequent weeks battling an ankle injury before joining Marshall on injured reserve in mid-November.