Unless most of the teams in your league carry a backup defense, more than half of the league should be available to you every week. Perhaps instead of playing one above average defense every week, regardless of matchup, we should instead target the widely available waiver wire defenses with the best matchups in any particular week? You can always make exceptions if your rent-a-defense gets hot - heck, you might find a team worth starting all year by accident. Even if you don't, good matchups will continue to get greater in number as more QBs get injured or yanked as the season goes on. You can steal some points from bad weather come December. Each week I'll list the startable fantasy defenses on most waiver wires and rank them.
**Philadelphia, New Orleans, and Baltimore have great matchups but are assumed to be rostered**
Minnesota (vs Tampa Bay)
This is the slam dunk pick of the week. The Vikings will unveil the Brian Flores defense, and by most accounts, it will be just as confusing and chaotic for opposing quarterbacks as previous versions in Miami and New England. Baker Mayfield is going to be tested early and often against a unit that will be trying to make big plays with deceptive blitzes that force the quarterback’s hand before they are ready to make a decision or otherwise are looking at something different than they saw pre-snap, or in pre-game preparation.
A Look Ahead: @PHI LAC @CAR - We will come back to them if Bryce Young doesn’t look ready early in the season.
Washington (vs Arizona)
If you can’t get Minnesota, Washington is a fine alternative. We don’t know who is going to start at quarterback for Arizona, which in and of itself is a good sign for the opposing defense. Washington will be playing in front of a home crowd, and they have the best defensive tackle duo in the league to ruin the Cardinals' offensive plans.
A Look Ahead: @DEN BUF @PHI - Maybe we’ll stick with Washington if they roast the Cardinals and Russell Wilson struggles against the lowly Raiders defense.