Problems on defense could be a real downer for the Washington Redskins. I’m still banking on the Redskins having their most successful season in more than 20 years, largely because their offense could be the NFL’s best. But the defense’s issues are mounting almost as fast as opponents sped past Redskins defensive backs last season.

The defensive line won’t be at full strength to start the season while end Jarvis Jenkins serves a four-game suspension for violating the league’s policy on performance-enhancing drugs. The loss of hard-hitting rookie safety Phillip Thomas, who suffered a season-ending foot injury in Washington’s preseason opener, weakens a position that was the team’s least productive in 2012. And with the apparent lack of depth at inside linebacker, the Redskins are crossing their fingers that 38-year-old London Fletcher can turn back the clock once more.

Although sharp defensive coordinator Jim Haslett is as upbeat as usual (“I really like our guys,” he says often), he also smiled a lot last season despite having arguably the league’s worst collection of safeties. Haslett never looks at a glass as being half-empty, and Coach Mike Shanahan definitely isn’t the pity-party type, either. Despite losing key players because of injuries and suspensions, the defense has enough talent to achieve its goals, Haslett and Shanahan believe. Some proof would be nice, too.

Preparing backups to fill bigger roles is a priority for Washington, which faces Pittsburgh in a preseason game Monday night at FedEx Field. The Redskins’ important work starts up front.

The Redskins were counting on Jenkins to emerge as a top-notch performer from the outset this season. Beginning his third year in the league, Jenkins shined in training camp in 2011 but missed his entire rookie season after having knee surgery. He took over as a starter in 2012 after Adam Carriker suffered a season-ending injury in Week 2 and was expected to provide stability with Carriker sidelined for this season as well because of yet another surgery.

The Post's LaVar Arrington, Mike Wise, Dan Steinberg and Mitch Rubin debate whether they would rather watch a Nationals game or a meaningless Redskins preseason game. (Post Sports Live/The Washington Post)

Plans changed on the second day of training camp: The NFL suspended Jenkins, who is permitted to practice and play in preseason games. The Redskins entered camp knowing that outside linebacker Rob Jackson, one of the team’s best pass rushers, also must sit out the first four games for violating the league’s substance-abuse policy.

To address the loss of Jenkins, the coaching staff moved nose tackle Chris Baker to end, and longtime backup lineman Kedric Golston figures to be in the mix more during the first month of the regular season against Philadelphia, Green Bay, Detroit and Oakland.

The main responsibility for ends in the 3-4 is to engage offensive linemen on running plays to prevent them from blocking linebackers, which could lead to long runs. Baker and Golston should be fine in that aspect of the job. But standout 3-4 ends also possess the athleticism to make their share of big plays. Jenkins has highlight-tape potential. The guys behind him aren’t showmen.

At safety, Thomas had recently displayed some nifty moves. After a slow start in his adjustment to the NFL, Thomas made up ground in camp. He would have played a lot this season, especially because veteran Brandon Meriweather — who appeared in only one game in 2012 — is slowly regaining form after knee surgery. In the past week, the Redskins increased Meriweather’s reps in practice. Still, he essentially sat out a whole season and is coming off major knee surgery. You shouldn’t pencil him in to have a Pro Bowl year.

If Meriweather is hobbled again, the Redskins would turn back to Reed Doughty and DeJon Gomes. Both players do their best work on special teams. You won’t find anyone more committed to the Redskins than Doughty, who has started 30 games over the past three seasons and is beginning his eighth year with the organization.

The Redskins, however, drafted Thomas and Bacarri Rambo, who starts at one safety spot, because they must get much better at safety. Haslett won’t receive the results he’s seeking if Doughty and Gomes play as much in 2013 as they did in 2012. The Redskins really need Meriweather to stay on the field. Same goes with Fletcher.

It is impressive Fletcher can even walk, let alone continue to play inside linebacker at a high level as he begins his 16th year in the NFL. Hamstring pain, a balance problem, an ankle injury — Fletcher overcame it all last season to extend his amazing consecutive-games-played streak to 240. He’s also the league’s active leader with 199 straight starts. The Redskins may have to lean on him much more than they would have preferred at this stage of his career.

The Redskins began preparing for Fletcher’s eventual retirement by drafting Keenan Robinson in 2012. But the succession process is on hold after Robinson tore a chest muscle in camp and is out for the season. Perhaps veteran backups Bryan Kehl and Nick Barnett would deliver if called upon. From the look of things at this point, though, the Redskins can’t afford to give Fletcher and his rising protege at the other inside spot, Perry Riley, many snaps off.

If the situation seems a tad bleak, remember: In 2012, the Redskins won 10 games and the NFC East title despite ranking 28th overall on defense and 30th against the pass. Rambo and rookie cornerback David Amerson will help the secondary. The return of outside linebacker Brian Orakpo should be a big boost for the pass rush, and Haslett was a play-calling whiz as the Redskins closed last season with seven consecutive victories.

Haslett will have to juggle things, especially in the first four games, but the good news for Redskins fans is that he has proved adept at problem solving. And no one said the road to 12-4 would be easy.

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