I drafted Shaun Alexander in the 13th round in 2001. He was a second year guy stuck behind Ricky Watters, but everyone said he had a lot of talent. I went on to a 15-2 record that year (no playoff system in that league back then).
If you thik a guy like Turner is good (and I do think he is very good), draft him and keep him on your bench. If nothing happens to the starter for a few weeks, you could always consider trading him at that point, too. A lot of times, you don't even have to make the offer, as the guy with Tomlinson will get nervous when he hears about a strained groin or bruised foot, and wants to go get the backup. You might get a lot more out of him at that point.
Or you could just keep the backup on your bench and wait for the starter to get injured. I was in a league in 2004 where the defending champ, drafting at the turn, took Manning and Harrison. He then got Curtis Martin (gold mine that year) in the 3rd round, and loaded up on WR's for the next 3 rounds. Then he started drafting everyone's handcuffs. He ended the draft with about 8 RB's, but only the one starter. Throughout the year, whenever a stud RB was injured, this guy could just plug in that handcuff and get starter-quality points. Eventually he made some trades for better starters and defended his championship successfully. It was a strategy I'd never seen before, and I'll never forget. Just something to think about.