Going from memory here so my details might be wrong....
Back in mid/late 80's Dickerson was holding out and I think Charles White was injured or re-habbing. We had an unknown running back in camp named Gerald Harris. He promptly ran for 100+ yards in each of our preseason games. I knew this guy would make the team. He didn't. I don't think he ever played another down in the NFL.
Since then I've been very wary about assuming a guy is a lock to make the team based on a pre-season game. Obviously I want the best guys on the team, but just because McCutcheon had a great night against some really poor Charger DB's. (How many PI penaties did they have?) I don't think we need to start worrying about who they have to cut to keep him just yet.