ALAMEDA, Calif. — Derek Carr is struggling, but the Oakland Raiders appear intent on letting the rookie quarterback work his way out of it rather than turn to their $8 million backup, Matt Schaub.

Carr completed 18 of 34 passes for 172 yards Sunday, but he couldn’t get the Raiders into the end zone in their 13-6 road loss to the San Diego Chargers. The Raiders have produced 233 yards or less on offense in four of the past five games, and they have just 17 first downs in their past two games.

What’s wrong? Carr has offered the same answer for the past month.

“That’s going to be hard to say without seeing the film,” Carr said. “I felt at times we were moving the ball well. There may be a missed assignment here or there, but there’s not just one answer to it.”

More than half of the Raiders’ drives this season ended in either a three-and-out or a turnover.

In the past two games against the Denver Broncos and San Diego, the Raiders went three-and-out 15 times. In each of those contests, Carr has played a role in an embarrassing turnover.

Against Denver, Carr, under pressure, flipped a short forward pass to guard Khalif Barnes, believing Barnes was running back Darren McFadden. Barnes made the catch but fumbled, and the Broncos recovered. Denver declined the penalty for an ineligible receiver touching the ball.

Against the Chargers, the rookie’s mistake happened on the first play from scrimmage.

From a shotgun formation, Carr stepped forward to change a play, only to be surprised by the snap from center Stefen Wisniewski. The ball hit Carr’s hands and fell to the ground, and San Diego linebacker Donald Butler recovered at the Oakland 22-yard line.

Two plays later, Chargers quarterback Philip Rivers threw a 22-yard touchdown pass to wide receiver Malcom Floyd. The game was 53 seconds old, but that was the only touchdown of the day.

“We had a certain play called, and I went up to change the play and it just happened to be snapped at that time,” Carr said.

Wisniewski said, “It was a miscommunication, and it shouldn’t have happened. If we don’t turn that ball over early, maybe we’re not even down because of how well (the defense) played.”

Carr already holds Raiders rookie records for passing yardage and touchdown passes, but has yet to get his first win. His play, as well as that of the offense as a whole, has regressed in recent games.

When the Raiders played the Chargers in Oakland on Oct. 12, Carr passed for 282 yards and a career-high four touchdowns. The Raiders led 28-21 in the fourth quarter, only to have the defense give it up to Rivers in Tony Sparano’s first game as Oakland’s interim coach.

Schaub was acquired from the Houston Texans to be the starting quarterback while Carr gained some seasoning. Schaub had some elbow issues and Carr struck for three touchdown passes in the preseason finale, prompting then-coach Dennis Allen switched to Carr. There was no looking back.

Apparently, there still isn’t.

“We never had that conversation on the sideline,” Sparano said when asked if there was any thought of going to Schaub against the Chargers.

Asked if he would consider playing Schaub at some point, Sparano said, “I don’t see that happening right now.”

REPORT CARD VS. CHARGERS

–PASSING OFFENSE: D — Take away a 33-yard pass to TE Mychal Rivera to set up a field goal, a 35-yard pass to WR Kenbrell Thompkins to set up a field goal and a late 28-yard completion to WR James Jones, and QB Derek Carr completed 15 passes for 76 yards. That goes a long way toward explaining six three-and-outs.

–RUSHING OFFENSE: D-minus — Back-to-back runs by RB Latavius Murray that totaled 37 yards in the third quarter provided the bulk of the Raiders’ ground game. Those plays stand as the team’s only two rushing first downs in the past eight quarters of play. RB Darren McFadden had eight carries for 21 yards; RB Maurice Jones-Drew finished with six carries for 6 yards.

–PASS DEFENSE: B — The Philip Rivers who sliced and diced the Raiders’ defense in the first meeting got the ball into the end zone on his first pass play 53 seconds into the game, then never again. The Raiders had a good interior pass rush, with DT Antonio Smith recording his first sack of the season. LB Khalil Mack also had his first sack and drew two holding penalties. The coverage was very good, particularly from CB D.J. Hayden. With a few takeaways, Oakland would have earned an “A” grade.

–RUSH DEFENSE: B — The Chargers had to work for their yards, rushing 32 times for 120 yards, a 3.8-yard average. Mack consistently beat whoever was in front of him and blew up blocking schemes. He had two tackles for losses. RB Ryan Mathews was the leading rusher with 70 yards on 16 carries, but 20 of those came on one run.

–SPECIAL TEAMS: C — PK Sebastian Janikowski converted both of his field-goal attempts, and Marquette King put two punts inside the 20-yard line, including one to the 1 with CB Chimdi Chekwa downing the ball. Neither return team produced anything impressive. There was nothing the Raiders could do about Chargers P Mike Scifres pinning them to the goal line.

–COACHING: C — Interim coach Tony Sparano has the Raiders playing hard and competitively, but the team is still sloppy at times, racking up penalties (eight for 41 yards) and inexplicable mistakes (Carr and C Stefen Wisniewski getting crossed up on a shotgun snap fumble). The defensive game plan was executed very well. However, whatever it takes to get Carr on track, offensive coordinator Greg Olson hasn’t figured it out.