Let's pump the brakes a bit. There's no reason for Tomlin to play coy with Wilson's availability. He strikes me as the type of guy who would have no issue naming Fields the starter.
Similarly, as we saw with CMC, sometimes a calf issue is teamspeak for an Achilles injury. That would explain the uncertainty surrounding Russell's availability. Honestly, this is the most likely explanation. By all accounts, Russell worked extremely hard this offseason to increase his mobility. We've seen this time and again with athletes over 30—getting in the best shape of their lives and immediately injuring their Achilles from all that added stress.
Either way, it's not a great sign for the future of his career.