When you're thinking about how a player will perform, you usually aren't thinking "this player will probably catch eight passes for 123 yards, I bet!" That doesn't really make sense unless you have the sports almanac from Back to the Future II; predictions about the future don't work like that. You're thinking "they might have a good game because of X!" It's a vague idea, which is sometimes hard to use when comparing players.
Here's how to use this effectively: figure out the player's ceiling and floor.
The ceiling is the best-case scenario for the player (and factor in about the odds of that scenario happening). Conversely, the floor is the worst-case scenario for the player (and factor in about the odds of that scenario happening).
Seems simple enough - and it really is that simple. Apply it for yourself and see what happens. We'll use Keenan Allen and Cordarrelle Patterson for our example.
Keenan Allen is the WR1 in SD and will be at home against the Seahawks. Last week he pulled down 5 passes for 37 yards on 9 targets while being aggressively
Given that, it's probably safe to set his floor somewhere around what happened in Week 1 to Allen (since he likely won't be blanketed all game, but probably some of it) - blanket coverage, but pulled in a number of shorter passes. His ceiling is probably a little above what Nelson did last week (9/83) with a possible score in there. I tend to think the ceiling is slightly more likely than the floor, but that's very open to differing opinions.
Cordarrelle Patterson only grabbed 3 passes for 26 yards on 5 targets (in comparison, Jennings had 7 targets), but also rushed for 102 yards and a score on 3 carries against the Rams last week. This week he faces the Patriots. The Patriots have Revis instead of Talib now, but didn't use him in shadow coverage in Week 1 against Mike Wallace/the Dolphins, indicating they probably won't use him in that capacity too often. Wallace deserves shadow coverage over Hartline more than Patterson deserves shadow coverage over Jennings, so it's probable - as probable as any Belichick prediction ever is - that Revis will be in the same capacity against the Vikings. The Patriots have a horrendous run defense and AD is AD, so lots of yards will be picked up in the ground game. Patterson's carries are super-unpredictable, because either AD's rushes cause the Patriots to pack the box allowing Patterson to run outside the tackles or AD's rushes are so effective that Patterson doesn't even get the minimal 3 carries he got last week. Regardless, 100 yards rushing isn't likely to happen again, so set that aside. The Patriots needing to pack the box means Patterson will be left open (anytime he's not dealing with man coverage from Revis) across the middle, meaning lots of potential opportunities there.
Given that, Patterson's floor is probably a little higher than Week 1 in terms of receiving totals. It's reasonable that the passing game might not be needed quite as much, but when it is used it'll be effective. I tend to think the Patriots will keep the game close at a minimum, so passing will remain part of the equation all game, raising the floor and making the floor less probable. It's not unreasonable to set the ceiling high, including a score and maybe a couple middling carries to round out the totals.
I have Patterson edging out Allen this week as a result, in large part because I think Allen's floor is more likely than Patterson's. That said, remember that essentially what we're doing is predicting a range of possibilities, and there's lots of overlap in that range - I have these two players very close in my rankings.
The more detail you can think through, the better, but even with limited information the same principle applies: figure out the realistic best case scenario, the realistic worst case scenario, about the relative probabilities of each, and use that to make your start/sit decisions.
Soon, I'm going to start posting a weekly Strictest Cornerback Watch to help decide how concerned you should really be about players like Sherman, Peterson, Haden, and other star corners.