CINCINNATI — Bengals defensive coordinator Paul Guenther climbed into his car after Cincinnati’s 42-21 home loss to Pittsburgh on Sunday and asked himself the same question most of the team’s fans wanted to know:

“What just happened?”

Guenther’s defense had done a solid job of holding the explosive Steelers in check for three quarters, helping the Bengals build a 21-17 lead. But 25 points and 229 yards later, Guenther was mumbling to his steering wheel.

“All I can tell you is I felt great, and then, all of a sudden, 12 minutes later, I’m in my car going home going, ‘What the hell just happened?'” Guenther said. “The wheels just kind of came off after they got the turnover. They converted on third down and scored on the next play. Then we give up the long pass and, before you know it, it’s a two-score game.”

The turnover was an Andy Dalton fumble on the Bengals’ 24-yard line just one play after the Steelers had closed to within 21-20. Four plays later, Le’Veon Bell smashed his way into the end zone for the go-ahead touchdown to accelerate the fourth-quarter avalanche.

“One of the messages I am going to give the guys, that I have been telling them, is we have to be better when the odds are against us a little bit,” Guenther said. “When things aren’t going quite as smoothly as we want them to go — it’s the NFL — we have to go out there and buckle up and play good defense.”

The ability to do that has been a struggle all year for a Bengals defense that entered Monday night tied for 28th in the league.

The 543 yards the team allowed marked a season high, but it was the third time this year Guenther’s group has surrendered more than 500.

It’s the first season in team history the Bengals have allowed three 500-yard games, and there are only two other instances where they gave up 500 twice in the same year — in 1968 and 1969, the first two years of the franchise.

Before this year, the Bengals hadn’t allowed 500 since 2007, when they surrendered 554 in a 51-45 loss at Cleveland.

Despite Sunday’s implosion, Cincinnati still has a half-game division lead on Pittsburgh and Baltimore with three games remaining, including a rematch in Pittsburgh in Week 17.

“We know what’s in front of us,” Guenther said. “We control our own destiny. We need to move forward. We’ve got to get better and move forward to the next opponent.”

NOTES: LB Vontaze Burfict (knee) missed his sixth consecutive game and there are questions about whether he will play again this year. … QB Andy Dalton missed one play after getting the wind knocked out of him and was later taken out when the score got out of hand, but he said he was fine. … CB Terence Newman left Sunday’s game twice, with a shoulder injury and later with an ankle issue, but returned both times. … CB Adam Jones left the game with a chest injury but returned. … RT Eric Winston, who signed with the team Dec. 2, made his season debut Sunday, playing 11 snaps in mop-up time in the fourth quarter.

REPORT CARD VS. STEELERS

PASSING OFFENSE: A — A career day for A.J. Green and a top-five passer rating for Andy Dalton should not result in a 21-point loss. Green’s 224 yards were the second most in franchise history and included an 81-yard touchdown in which he put various talents on display, beating one defender, deceiving another and slipping the tackle of a third. Dalton completed 72.4 percent of his passes (21 of 29) and threw multiple TD passes without an interception for the first time this year. And he didn’t just play pitch and catch with Green all day. Tight end Jermaine Gresham had three catches and a TD and backs Hill and Bernard also caught three passes.

RUSHING OFFENSE: C — Dalton’s 20-yard touchdown run on a read-option play essentially was canceled out by Dalton’s fumble on the same play in the fourth quarter. Yes, it led to the onslaught that resulted in the blowout, but that’s not on the run game. That’s on the defense. The run game was average all day and finished with 86 yards on 21 carries. But Jeremy Hill (8-46) and Giovani Bernard (6-17) were limited by both the Pittsburgh defense and the Bengals’ explosive passing game.

PASS DEFENSE: D-minus — You’re not going to have much success against any quarterback if you don’t pressure him or cover on the back end. If the quarterback is Ben Roethlisberger and that’s your M.O., it’s going to be lethal. And it was. The Bengals not only failed to register a sack, they had only one quarterback hit all afternoon. And they recorded only four passes defended against Roethlisberger’s 39 attempts. Roethlisberger averaged 9.0 yards per attempt, a number inflated by a 94-yard, back-breaking bomb that gave the Steelers a two-touchdown lead with a little more than eight minutes remaining.

RUSH DEFENSE: D — There were stretches where the Bengals were able to limit Le’Veon Bell to short, ineffective runs. But the cumulative result was a mess as Bell went for 185 yards and ultimately averaged 7.1 per attempt. The Steelers had 193 yards on the ground, the second most allowed by a Bengals defense that had appeared to turn the corner during the three-game winning streak on the road. Pittsburgh was able to convert 8 of 16 third downs in large part because Bell set them up with manageable distances.

SPECIAL TEAMS: B-minus — Kevin Huber continues to be the only consistent piece on the team. He averaged 46 yards on his seven punts, including a 44.0 net. Huber put the defense in position to alter the course of the game by pinning the Steelers at their own 6 midway through the fourth quarter, only to watch Pittsburgh throw a 94-yard touchdown on the first play. Some of the other special teams elements were a bit of a wreck. Cedric Peerman fumbled on a fake punt attempt, and Adam Jones dropped two kickoffs in the end zone when he appeared ready to bring them out, and he also failed to catch a punt and nearly turned it into disaster as the bouncing ball came dangerously close to hitting him. The coverage units were solid, holding the dangerous Antonio Brown to a 4.7-yard average on three punt returns while locking down Marcus Wheaton for a 19-yard gain on the only Mike Nugent kickoff that didn’t result in a touchback.

COACHING: C-minus — The fake punt on the opening drive was a strange call. While it made sense to try to get a first down at the opponent’s 40 rather than risk a 20-yard net with a touchback, why not just line up the offense and go for it on fourth-and-3? There have to be a least a dozen plays in Hue Jackson’s book that are better options for converting three yards than trying to fake Cedric Peerman through the middle of the punt defense. The defensive game plan was too timid against a quarterback of Roethlisberger’s ability. He’s going to make plays no matter what, so why not make him pay for a few of them every now and then? His easy stroll through the first quarter probably played a role in how strong he was in the fourth quarter. And speaking of the fourth quarter, you don’t take a four-point lead into it and then lose by 21 without the coaching staff absorbing a chunk of the blame.