Even with Plummer at QB you have to consider Denver a contender this year. They have built a pressure defense to compliment their productive offense, which improved last season thanks to Plummer minimizing his mistakes.
Shanahan has had success against us, but our history against Denver is overstated. We've played them 5/6 in Denver, an extremely tough place to play (see Shanny's 74-20 home record). Surprisingly, Belichick is actually the only active coach to have 2 wins in Denver against Shanahan.
Our game 9/24 will definitely measure how good we are going to be this year. A good running attack and the use of screens should neutralize their blitzes (a better o-line would help too). Our D is capable of shutting down their offense (see playoffs). We just have to put together a complete game, something we never seem to do against Denver.