SANTA CLARA, Calif. -- During the offseason, the San Francisco 49ers wagered heavily that their defense could take a step forward in 2018 mostly on the strength of players who were already on the roster.

Instead of investing big dollars in free agency or valuable capital on high-end draft picks such as Derwin James or Minkah Fitzpatrick, the Niners bet big that their defense -- loaded with many starters either coming off serious injuries like defensive lineman Arik Armstead, safety Jaquiski Tartt and linebacker Malcolm Smith or lacking in proven production or both like cornerback Ahkello Witherspoon, free safety Adrian Colbert and defensive lineman Solomon Thomas -- could be good enough to complement quarterback Jimmy Garoppolo and the offense. So the only significant additions to the defense came in the form of cornerback Richard Sherman and third-round linebacker Fred Warner.

Given how coordinator Robert Saleh's Cover 3 defensive scheme has worked well in a handful of other NFL locales, it wasn't crazy to think continuity would be a boon for the defense, especially if Garoppolo and Co. provided enough offensive fireworks to cover for any growing pains.

"I don’t think this scheme is going anywhere but it’s about each individual player understanding the weaknesses and strengths of the defense and I think that comes with time, that comes with experience and growing together and continuity,” Sherman said “There isn’t a ton of continuity on this defense right now."

Indeed, the 49ers' bet on stability hasn't brought much of a return so far.

While you don't have to look hard to find numbers that indicate a defense that has been good at times (the Niners are 11th in yards allowed per game and ninth in yards allowed per play), it's also pretty easy to find numbers that show the Niners need much more from their defense, especially with Garoppolo lost for the season to a knee injury.

Most notably, the 49ers simply aren't getting enough big defensive plays, allowing offenses to put together long drives that usually end with points. Five weeks into the season, San Francisco is last in the NFL in takeaways with three and their nine sacks are tied for 24th in the NFL.

Taken by itself, the lack of big plays the Niners have yielded (hence, the top-10 rank in yards per play allowed) is a good sign for the Niners, but it's also not the best indicator of performance. Saleh's scheme, which is rooted in what Pete Carroll has done in Seattle and is now run by the likes of the Jaguars, Falcons and Chargers, is designed to force opposing offenses to methodically work their way down the field. The belief is that it's difficult for an offense to execute so consistently against a strong pass rush, and a ball-hawking secondary will eventually force mistakes.

The problem, however, is that they don't have the pass rush or the secondary to force such errors. For anyone paying attention, the lack of pass rush outside of defensive tackle DeForest Buckner should be no surprise. The Niners didn't address edge-rushing in any meaningful way and hoped Thomas would take a step forward playing outside even though he's looked better on the interior.

Because none of that has happened, the 49ers are generating a disrupted drop-back (a sack, batted pass, pass defended or interception) on just 11.2 percent of opponents' drops, last in the league. The Niners are also contacting opposing quarterbacks on just 8.6 percent of drop-backs, which ranks 30th in the NFL, and sacking opposing quarterbacks on 4.7 percent of drop-backs, which is 27th.

The inability to generate a pass rush with just the front four has forced Saleh and coach Kyle Shanahan to blitz a bit more recently, though even that has been spotty because some of those blitzes have resulted in big plays ... for the other team.

“This whole league is based on explosive plays, and the elimination of explosives make offenses earn their paycheck," Saleh said. "Yes, pressuring is awesome at moments, but if you live by the sword, you die by the sword. If you continue to pressure and you continue to be aggressive, you’re going to pay a price."

The Niners have sent five or more pass rushers on 17.6 percent of opponent drop-backs, which is still the sixth-lowest number in the league and well below the league average of 24.3 percent. For comparison, the Niners blitzed 23.6 percent of the time last year, which came in a little below the league average of 27.6 percent.

In part because of the opponent and in part because of a lack of pass rush, the Niners have been blitzing more of late. They blitzed Arizona rookie quarterback Josh Rosen 32 percent of the time last week and the result was the defense's best outing of the season. That won't be so easy against notorious blitz-killer Aaron Rodgers this week or the high-powered Rams the following week.

“When everything’s perfect, you can just sit there and play Cover 3 every single play, get home, have your four guys underneath drop, make them check it down... That makes life very easy and it makes it very hard for an offense to get to. When you don’t have all of that firing and it’s not going as well as you want, you have to do other stuff."

Aside from pass-rush issues, the Niners defense has also dealt with injury problems, particularly in the secondary where Sherman, Tartt, Colbert, Witherspoon and Jimmie Ward have missed full games or large chunks of them.

That makes it hard to be consistent. The 49ers rank last in the league in interceptions per pass attempt, tied for 19th in pass breakups and 21st in opponent passer rating. The Niners are also last in the NFL in turnover margin (minus-8), a number that's skewed by the offense's five-giveaway game last week but also by the defense's inability to create takeaways of their own.

As this season seems set to slip further away, the 49ers can hope that their young talent will get healthy and continue to develop. But it's hard to envision a scenario in which upgrading the edge rush and the secondary won't be at or near the top of the offseason priority list. Adding those pieces would go a long way in establishing the cohesion the Niners seek and offer more of the big plays they're so desperately missing.

"When you play those type of schemes, the longer guys play together, the better they do," Shanahan said. "That was one of the best things Seattle had going for them. You can see that with Jacksonville. It’s a great scheme. They’ve got some good players, but it takes time for 11 guys to work together.”