Image Credit: Andrew GiesemannThe professional football free agency period feels like it happened 500 years ago. Indeed, staying at home to avoid a silent, invisible enemy can fray the nerves and thin the grey matter in your skull.The San Francisco 49ers traded defensive lineman DeForest Bucker to the Indianapolis Colts on March 18, in exchange for the Colts’ first-round draft choice in 2020.On the outside, the 49ers’ defensive line was as dense as a neutron star. It has been the best unit on the roster for at least four seasons. But even the most reliable parts of a team do not always remain. Even the most gifted sorcerer could not find the formula to keep four first-round draft choices together through the 2020 season and beyond.Buckner’s departure does impact the defense, but with the talent remaining, it is possible to limit the ripple from becoming a tidal wave.Defensive LineAs you probably know, defensive coordinator Robert Saleh utilizes a base 4-3 over and under front. You can tell the difference between an “over” and “under” front by the position of the interior defensive lineman. This fall, when the defensive line takes its stance, watch where Solomon Thomas or D.J. Jones align.The formation is over when Thomas or Jones shades to the strong, or closed, side of the offensive formation. Under is the opposite; Thomas or Jones will shift slightly to the weak, or open, side.As much as I detest guessing the team’s starting line up in the spring, we need something to start a basis for a discussion. So, with great hesitation, I will assume the front four look like this:Edge: Nick Bosa3i: Arik Armstead1-3: Solomon ThomasEdge: Dee FordLet’s start at the edges. Saleh still has his two superhuman edge defenders in Bosa and Ford. These two marvels will continue to force the opposition into no-win situations. The speed and strength displayed by both men are too much for one tackle or running back.Further, the use of the wide looks, no matter on the weak or strong side, place a tackle or tight end into one-on-one situations against superior athletes. Betting against Bosa and Ford is akin to splitting 5s or 10s on a blackjack table.It’s not easy for an offensive line to draw up pass protections that shore up the edge with a double-team block. Further, it leaves the offense with one less receiver and could open gaps on the line.For example, a “Tug” protection tells the tackle and guard to exchange assignments; the tackle blocks down, and the guard steps around to take on a linebacker or defensive end.A “Jet” protection slides four linemen left or right and leaves the tackle and back alone to protect the opposite side. It’s an excellent pass protection and a staple in head coach Kyle Shanahan’s offense. Even if the back and tackle double up the edge, it leaves the remaining offensive line in one-on-one battles against Armstead and Thomas.Expect Armstead to command more double-team blocks on runs or in certain pass protections, which then leaves Thomas in the situation to beat his man to disrupt a lead block or sack the quarterback.The thought of Thomas finally becoming a significant contributor to the 49ers’ defense is as thin as a windy lunar day. However, if he makes it through training camp and does not get traded, this will be a make or break season for him.LinebackersLinebackers and defensive linemen have a symbiotic relationship. Great linebackers are made into legends when the trench men stuff gaps and snuff out lead blocks to allow a rabid linebacker to identify his key and make a stop. These are small nuances often missed in a live game, but they are critical for linebackers Kwon Alexander and Fred Warner.Successful linebackers honed in on their assignments, then result in a fully functioning defense.The loss of Buckner and what he did for Alexander and Warner is not yet known. We may see no disruption or decline in play.I am interested to see if the starting four linemen can clear blitz paths for Alexander and Warner, if only for selfish reasons. I’d like to see Warner finish the season with more than five sacks.SecondaryIn the waterfall of interviews and stories leading up to the Super Bowl, one quote that some missed was veteran cornerback Richard Sherman’s comment regarding Saleh’s defense and secondary issues during the 2018 season.“He’s [Saleh] calling a lot of the same plays. He’s scheming it up just as he always has. I guess he has more talent, and I guess people are executing the calls that he calls. That was one of the things where I would get frustrated with his criticisms because people were like, ‘oh, my God, he’s calling a terrible game.’ I was like, ‘well, he’s calling a great game and poor execution more than anything.’ You call a blitz, and they don’t blitz. You call a cover two, they play cover three.”Whether Buckner was on or off the field for the mistake-laden season of 2018 is insignificant; what matters is the 49ers’ secondary tightened up considerably in 2019 and had help from the edge rushers forcing quarterbacks into bad situations.Keep in mind that the 49ers’ 2018 pass rush tallied a whole 275 quarterback pressures of any kind, resulting in a whopping two interceptions. Compare the same statistics with the 2018 Chicago Bears defense that had 342 total pressures of any kind and led the NFL with 27 interceptions. I trust the 49ers’ secondary will remain one of the league’s best, but it still needs the help of a defensive line and the linebackers. There is not a cornerback in the league that can cover a wide receiver for seven or eight seconds and expect to make a play.The man who may feel an enormous wave hit him is Saleh. We don’t have an inkling as to what his plans are for his defense or if he plans to convince general manager John Lynch to make a trade for an upgrade somewhere on the roster.You can follow Bret on Twitter here!Stay tuned to 49ersHub for more great 49ers coverage and analysis!