It's now or never for these Chargersby Alex MarvezSeeing two longtime teammates call it quits this offseason gave San Diego Chargers quarterback Philip Rivers a sobering reminder of his NFL mortality.“What hit me right in the face was this doesn’t last forever,” Rivers said about the retirements of running back LaDanian Tomlinson and guard Kris Dielman. “This does come to an end.”As will the tenures of head coach Norv Turner and general manager A.J. Smith if the Chargers don’t rebound in 2012.Their seats can’t get much hotter after two straight seasons without a playoff appearance and the failure to reach a Super Bowl in previous years with more talented rosters. If Turner and Smith were fired, a roster shake-up would likely follow under a new football regime.This fact isn’t lost on Chargers players as the team opens its preseason.Rivers described his locker room as having a “backs-against-the-wall mentality.” Wide receiver Vincent Brown seconded the thought, saying, “We can’t go out like we did last season.”“We’re driven by not making the playoffs, not playing to our standard and not playing consistent,” free safety Eric Weddle told FOXSports.com after a weekend practice. “We love our coach. We love this organization. But we don’t know what the future holds. All we can worry about is each day, each meeting, each practice and getting the most out of now.”The sense of urgency is reflected by the uncharacteristic way Smith has overhauled San Diego’s roster.Loathe to dip into free agency during his 10 previous seasons in San Diego, Smith added a whopping 17 veterans during the offseason and re-signed two starting offensive linemen, left tackle Jared Gaither and center Nick Hardwick. Most newcomers will serve in backup or contributing roles — “layering” is how Smith described it — as he tries to address the depth problems that crippled the team’s playoff hopes in 2011.Smith said he realized such a radical personnel departure was needed last fall as the Chargers became mired in a six-game losing streak. The offensive line was particularly hard hit as 13 different players were used during that stretch.A 9-7 record would have given San Diego the AFC West title. Instead, the Chargers went 8-8 and finished behind Denver (8-8) as a result of divisional tiebreakers.“In hindsight, maybe if I had done a better job in some backup situations, instead of six (losses) we do three,” Smith said. “You do the math.”There is other blame to go around.San Diego had the NFL’s worst third-down defense, allowing the opposition to convert at a 49 percent clip. The Chargers fired coordinator Greg Manusky and promoted linebackers coach John Pagano to the position.Turner said the Chargers have tweaked their scheme to become “more aggressive up front.” San Diego added free-agent Jarret Johnson (Baltimore) and 2012 first-round draft pick Melvin Ingram to a group of pass-rushing linebackers that include Antwan Barnes, Shaun Phillips and Larry English. The Chargers also are expecting fellow rookie Kendall Reyes to contribute as a pass rusher in the defensive-line rotation.“When you have a great pass rush, the (defensive backs) are even better,” Chargers inside linebacker Takeo Spikes said. “It goes hand in hand. Everything starts up front.”Although offense is a Chargers strength under Turner, there was some tweaking there, as well. Rather than attempt to match the five-year, $55.6 million contract that Vincent Jackson inked with Tampa Bay, the Chargers signed four veteran wide receivers, led by Eddie Royal (Denver) and Robert Meachem (New Orleans).Neither fit the bill of tall wide receivers that the Chargers have featured in recent seasons, such as Jackson and the returning Malcolm Floyd. Royal and Brown are instead expected to serve as the top underneath threats to help spring Floyd, Meachem and tight end Antonio Gates for more downfield opportunities. Third-year running back Ryan Matthews also is expected to cede his third-down role to free-agent pickup Ronnie Brown. Matthews, though, has set the bar high for his rushing totals coming off a 1,091-yard season.“The first thing I asked him was, ‘We going for 1,800 (yards)?’ ” new Chargers fullback LeRon McClain said. “He said, ‘2,000.’ Our goals are set high.”For the Chargers to reach theirs, Rivers must rebound from what for him was a subpar 2011 campaign. Rivers remained among the league leaders in passing yards (4,624) and touchdowns (27), but he also threw a career-high 20 interceptions and lost five fumbles.“I know I need to play better,” Rivers said. “We can talk about every stat in the book, but it’s just don’t turn the football over. There was a lot of good last year, but the bad was real bad.“I’m not going to go play careful this year, but it’s a mind-set of when to fit it in there, when to check it down, and when to throw it up in the bleachers and let Mike Scifres punt it 70 yards. It’s just playing the situation better.”At 30 years old and entering his ninth NFL season, Rivers is the best active quarterback yet to win a Super Bowl title. Rivers believes all of San Diego’s offseason moves give him the opportunity to shed that label.“We’ve had a great nucleus of guys here,” Rivers said. “We’ve been able for the most part to keep that intact. But I think it’s a matter of just trying to create a way to give you an edge. Maybe it’s a focus. Maybe it’s a mind-set. Maybe it’s a few different players here and there.“The one thing that stands out to me: In this locker room right now, there are probably more guys in here who love to play football than we’ve had in years past. That’s not to say that we haven’t had guys who loved it. We have. But there’s more of them. The more guys you have like that, the better chance you have to win.”That’s something the Chargers must do or more changes are coming.Alex Marvez and cohost Gil Brandt interviewed Philip Rivers, A.J. Smith, Norv turner, Takeo Spikes, Vincent Brown and LeRon McClain on SiriusXM NFL Radio.