What the heck is going on with Bears CB Kyle Fuller?

With his helmet still on and his hands tucked into his shoulder pads, cornerback Kyle Fuller took in the final minutes of Sunday’s game on the sideline.

He was a starter-turned-spectator — a frustrated symbol of how bad the Bears’ defense was against the Arizona Cardinals, who were merciless in a 48-23 victory at Soldier Field.

‘‘You’ve just got to expect better from yourself, as a group,’’ Fuller said. ‘‘You’ve just got to go in this week and put the work in.’’

Fuller has to and then some. He was replaced by Terrance Mitchell halfway through the fourth quarter. After the game, Fuller kept most of his answers to one word, doing his best to suggest Mitchell’s playing time wasn’t a message to him from his coaches.

‘‘It was nothing like that,’’ he said.

But it was a benching. The rest of the starters in the secondary stayed in, and linebackers Pernell McPhee, Shea McClellin and Christian Jones and other regulars played on. Fuller, who later played on special teams during a Cardinals extra point, said he was healthy.

Is his confidence shaken?

‘‘No.’’

Was being replaced frustrating?

‘‘No.’’

Does he expect to start next week?

‘‘Yeah.’’

Fuller’s problems were ugly. In the first quarter, he was called for pass interference on receiver John Brown after swiping at Brown’s arms early and failing to turn his head to the ball. It resulted in a 42-yard gain for the Cardinals, who scored two plays later.

‘‘It was one of those plays,’’ Fuller said. ‘‘They’re going to make their judgment call, what they saw.’’

In the second quarter, Fuller stumbled in coverage against Cardinals veteran Larry Fitzgerald, who made a diving eight-yard touchdown catch.

A more glaring gaffe came in the third quarter when Fuller was caught staring into the backfield on a flea-flicker. That left McClellin to chase Fitzgerald, who made a 28-yard touchdown catch.

‘‘Yeah, I should have been back there,’’ Fuller said. ‘‘It was just kind of one of those plays where you’ve got to do better.’’

The Bears were counting on Fuller to be a lot this season — a confident defensive leader, an every-game playmaker, a building block. Coach John Fox and defensive coordinator Vic Fangio sang his praises early on, but the first two games don’t support it. His performances in the preseason didn’t exactly meet the Bears’ high expectations for him, either.

Fuller’s strong start to last season — three interceptions in his first three NFL games — are a distant, fading memory.

A reliable pass rush would help, but Fox sounded ready for a competition for Fuller’s spot. Fuller didn’t break up a pass and was credited with just one tackle.

Fitzgerald finished with eight catches for 112 yards and three touchdowns. His final score went straight through Mitchell, who was violently stiff-armed.

‘‘We’re still evaluating,’’ Fox said of replacing Fuller with Mitchell. ‘‘We’ll evaluate every game as we march through the season. I just know we gave up 170 yards of penalties on downfield throws. And it wasn’t a mystery that we were going to see some of those.’’

Cornerback Alan Ball also was called for pass interference on Brown, which turned into a 38-yard gain for the Cardinals and eventually seven more points. But Ball’s issues aren’t as disconcerting as Fuller’s. Ball wasn’t the 14th overall pick in 2014. If Fuller’s failures continue, you can add more to general manager Ryan Pace’s already overflowing plate.

‘‘I don’t make roster moves, but I’m confident that we have the guys here that can play for them,’’ Ball said. ‘‘At times, we’ve showed that. At times, we haven’t.’’

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Email: ajahns@suntimes.com