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"How do you know when to give up on a guy in fantasy?"

It's a simple yet profound question, and one that was posed to me by someone who specifically wanted to know when he could safely cut Ricky Williams. The answer seem obvious, but it isn't as simple as it first looks. Of course production, injuries, matchups, underperformance and demotions all play into fantasy value and ownership. But this early in the season, it can be difficult to give up on a guy given the small sample size of evidence. It can also become increasingly difficult to bear unproductive bums on your team.

The fact is that the market changes quickly in fantasy. What would seem like crazy lineup questions in August -- "Who should I start this week; Carson Palmer or J.T. O'Sullivan? Matt Forte or Joseph Addai?" -- are now legitimate queries.

On the defensive side, we face the same issues. How much longer can you trust Julius Peppers and his zero sacks when a guy like John Abraham is still available in your league? Too often in fantasy football, we invest in past production and supposed upside rather than what a guy offers you now. In IDP leagues especially, utilize the waiver wire and get the most out of your roster every week, even if it means making some bold decisions, like adding some of the names below at the expense of the guys you thought you could trust.

Consider using these players if you are in immediate need of impact defenders, especially in deep leagues.

Linebackers and Linemen: Despite leading the league in sacks, the aforementioned John Abraham is only owned in 54 percent of ESPN leagues. Add "Big Abe" immediately as he's looking like the Pro Bowler he was in his Jets days. … The versatile Thomas Davis played safety and linebacker as a Georgia Bulldog and he's finally coming into his own as an outside 'backer for a resurgent Panthers defense. Consider Davis for immediate tackle production with some added turnover potential once the defensive line warms up. … Adewale Ogunleye and Alex Brown are the real catalysts in the Bears' defense becoming a force again. Owned in a paltry 5.3 percent of ESPN IDP leagues, Brown has a sack in each game so far and is an underrated double-digit sack threat to target. … Superstar menace to quarterbacks DeMarcus Ware gets the bulk of the pub in Dallas, and deservedly so, but fellow outside rusher Greg Ellis is a valuable addition in 10-team leagues given his penchant for big sack outings and dual eligibility at defensive end and outside linebacker. … There was growing concern over whether Dwight Freeney would ever be healthy enough to play up to his gaudy contract, but with a fast start to the season he's beginning to subdue doubters. Check if he's on the wire in your league, as the former Pro Bowler is available in almost half of ESPN IDP leagues. … The Colts defense has underperformed to date, but outside 'backer Freddy Keiaho is an emerging young player on a relatively thin front seven. In leagues that highly reward tackles, Keiaho is a player to pursue on both the wire and via trades. … Once considered a bona fide IDP superstar, Jacksonville's Mike Peterson is now a waiver wonder to consider. As long as Peterson is healthy and manning the middle for the Jags, he's deserving of a spot on fantasy teams. … With Zach Thomas wearing the Lone Star on his helmet, Channing Crowder is now the enforcer in the middle for the Fins. Against what should be a run-heavy Patriot offense, Crowder is a safe bet for double-digit tackles this week. … Kyle Vanden Bosch is a supremely underappreciated fantasy commodity and merits ownership in nearly any IDP format thanks to his sweet, sack-friendly schedule.

Defensive backs: Donte Whitner may finally deliver a huge season and live up to his big rookie deal. Both he and his surrounding defense are now healthy, and he's a leader on what is a vastly underrated secondary in western New York. … Former IDP star Charles Tillman is available in 60 percent of ESPN leagues after battling injury for parts of the last two seasons. Tillman had a pick last week and is the rare high-tackle corner that can afford his fantasy loyalists safety-like numbers. … Ken Hamlin is the Cowboys' best safety even though Roy Williams gets more face time. This week's matchup with the Packers should lead to a busy day for "The Hammer." … Playing alongside a promising Reggie Nelson, Brian Williams has been racking up linebacker-esque tackles in what has long been a productive strong-safety role in Jacksonville. … On what is becoming a feared defensive unit, safety Chris Hope is yet another Titans defensive back that deserves to be on your fantasy roster. An IDP superstar in 2006, a return to elite form is possible for Hope.

Target these guys for depth and bye-week plug-ins with an eye on their potential to be full-fledged starters.

Linebackers and Linemen: Consider Gaines Adams a Lee Evans type for the defensive side of the ball, being that he's the ultimate boom-or-bust option. One week he'll have multiple sacks; the next not a single tackle. That being said, Adams and fellow Tampa defensive end Greg White are solid plays to consider versus Chicago this week. … Dewayne White got off to a hot start last season and he's repeating the feat again this year with a sack in each of the first two games. While he's apt to disappear statistically from time to time, facing a 49er offensive line that has allowed an NFL-worst 12 sacks could make for some great numbers from the little-owned Lion. … Outside linebacker Bryan Thomas has seen his ownership in ESPN leagues nearly triple this week after posting consecutive big games for the Jets' new-look defense. … Champ's bro -- and now teammate -- Boss Bailey hasn't lived up to the hype yet as a game-changing 'backer, but he's getting an unexpected starting shot and faces a nice matchup this week against a short-pass-happy Saints offense. … If Pat Thomas can hold down the middle in KC all season, triple-digit tackles are a near-given. Thomas is an ideal flier in keeper leagues since there's no middle 'backer of the future in KC behind him.

Defensive backs: While Kerry Rhodes is supposed to be the proven fantasy stud, fellow Jets safety Eric Smith is the more productive player so far. Available in the vast majority of ESPN leagues, Smith is a nice depth player to target in what has been a slow start for many of the top defensive backs so far this season. … Mike Adams takes Sean Jones' place while the top IDP talent recovers from a knee injury. Adams should be a great tackle value for the next month while Jones rehabs. … Green Bay's Nick Collins has good potential this week for tackles against a juggernaut Dallas offense. … Rookie DB Tyrell Johnson is filling in for the injured Madieu Williams, and provides quality short-term production with long-term potential if Williams' neck doesn't heal properly. … Michael Griffin is emerging as a nice fantasy option on an IDP-rich Tennessee defense. Consider him a great spot start this week if you're struggling with underperforming defensive backs. … In abyss-deep leagues, target Skins safety Chris Horton, even if he returns to the nickel spot this week.

Jim McCormick is an analyst for ESPN.com fantasy football.