SANTA CLARA, Calif. - How bad has the 49ers defense been on third down?

Opponents are converting a league-worst 51.61 percent of their tries. And that number does not include penalties, where San Francisco also ranks dead last.

The 49ers have given opposing offenses eight first downs via penalty on the penultimate down, which increases that number to 61.5 percent (24 of 38) through the season’s first three weeks.

Not good.

What’s more: on third-and-6 or less, opponents have converted on 10 of 13 tries when calling passing plays. With Chip Kelly’s up-tempo Eagles offense on deck Sunday, the 49ers will have to find ways to get off the field to avoid being run ragged by the former Oregon coach’s no-huddle approach.

But according to 49ers defensive coordinator Vic Fangio, the potency of Philadelphia’s offense lies beyond the tempo.

”I think the no-huddle kind of camouflages for most people how good the offense is even without the no-huddle,” Fangio said. “They have a sound running game. It’s a very simple running game, but yet very effective. They give you a lot of different formations.”

Slowing down running backs LaSean McCoy and Darren Sproles to force third-and-longs will be one of Fangio’s mandates this week, along with plugging leaks in coverage that allowed the Cardinals two explosive touchdowns to speedy receiver John Brown. The Eagles have a similar player in Jeremy Maclin, who had eight catches for 154 yards in last week’s win over Washington.

Nick Foles, coming off a 325-yard performance Sunday, leads the league in yardage and is tied for third for yards per completion (13.0).

The 49ers problems against the pass, at least in the early going, are across the board.

”You can’t play good pass defense in this league without a good pass rush. They go hand-in-hand. You can’t have a good pass rush without good coverage,” Fangio said.

The 49ers have just four sacks through three games; four coming from lineman Justin Smith. And they blitzed more Sunday, but came away sack-less for the first time in 2014.

According to Pro Football Focus, San Francisco blitzed on 14 plays Sunday in an attempt to make Arizona QB Drew Stanton uncomfortable after blitzing a combined seven times in the first two weeks. But Stanton was comfortable in the pocket, completing the comeback from eight points down and outscoring the 49ers 17-0 in the second half.

How do the 49ers fix the pass rush?

That’s the question Fangio is trying to answer. It could come in a change in personnel with the insertion of second-year player Tank Carradine, who has played just one snap to this point.

Having played defensive end in a 4-3 during his collegiate career at Florida State, Carradine has added weight to kick inside to fit the 49ers’ 3-4 scheme. The learning curve has been steep for last year’s second-round pick and the numbers game has been against him to this point. Tony Jerod-Eddie and Demarcus Dobbs have been the primary backups to Smith and Ray McDonald.

”Special teams factors in, versatility factors in,” Fangio said when asked about Carradine’s scant usage. “How will they play in the base and the nickel? Right now, factoring all the things in, last week he wasn’t one of those six (active defensive linemen).”

In the secondary, it looks as though San Francisco will be without starting left corner Tramaine Brock (toe) for the third straight week. That means Perrish Cox will continue to play in his stead, forcing rookie Jimmie Ward to remain as the team’s nickel corner.

Ward has been picked on the last two weeks. He allowed all three of Brandon Marshall’s touchdowns in Week 2’s loss to the Bears and Brown’s go-ahead score last week in Arizona. He will likely have to go against fellow rookie Jordan Matthews, who made a pair of touchdown grabs in Sunday’s win from the slot.

Patrick Willis, who did not practice Wednesday and showed up on the injury report with a wrist issue, said he plans on playing Sunday, and knows the defense has to improve for the 49ers to avoid losing three straight games for the first time under Jim Harbaugh.

”There’s no question any time you play against an up-tempo offense, it’s going to wear on you a little bit,” Willis said. “But that’s why it’s important for us to be ready to get off the field on third down.”

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