They fell flat on their faces, losing 34-13 behind an embarrassing 24-point outburst late in the third and early in the fourth quarter.
For the Bengals, the win improved them to 8-6, keeping them, at the moment, in the lead for the AFC's final wild-card spot. The Eagles dropped to 4-10, their ninth loss in their last 10 games following a 3-1 start.
Read on to see the winners and losers for the Eagles from this Week 15 matchup.
The Eagles rookie quarterback was the hero last week against the Buccaneers.
Nick Foles completed a rookie-record 32 passes for 381 yards and two touchdowns, including a come-from-behind walkoff one-yard touchdown to Jeremy Maclin to give the Eagles an improbable 23-21 victory.
But Foles took a big step backward against the Bengals, completing just 16-of-33 passes for 181 yards, a touchdown and an interception. He also lost a fumble and posted a dismal QBR of 18.6.
The Eagles needed a bounce-back game from rookie running back Bryce Brown, who carried 12 times for just six yards last week.
They didn't get it.
Brown carried 16 times for 34 yards, an average of 2.1 yards per rush. He also lost a fumble, giving him four fumbles (all lost) in his four starts since LeSean McCoy suffered a concussion against the Washington Redskins.
Brown has shown that he has the ability as a runner to play in the NFL, but he will need to cut down on his fumbles if he wants to become a viable No. 2 option.
Third-year wide receiver Riley Cooper has played well since the loss of DeSean Jackson for the season.
Against the Bengals, Cooper caught three balls for 20 yards and a score, giving him a pair of touchdowns in the last three games.
He also collected two special teams tackles.
Philadelphia's backup tight end Clay Harbor found himself in a starting role on Thursday night with Brent Celek out with a concussion.
The game marked the first of Harbor's career, and it wasn't a memorable one. He caught three passes for 30 yards, a respectable total. However, he lost a fumble—one of four on the night for Philadelphia.
Playing against a Bengals defense that has collected sacks in bunches this season, the Eagles offensive line protected Nick Foles, allowing just one sack for three yards.
Third-year defensive end Brandon Graham was easily the brightest spot for the Eagles defense on Thursday night. He collected 2.5 sacks, a forced fumble and six tackles (one for a loss) against the Bengals.
Graham has thrived following the release of Jason Babin. He has collected four sacks in the last three games and has played his way into a starting role in the 2013 season.
Rookie first-round draft pick Fletcher Cox collected 1.5 sacks, four tackles (one for a loss) and two quarterback hits against the Bengals on Thursday night.
Cox has now collected a sack in two straight games. He had a rough stretch to the season following a hot start, but he is starting to hit his groove again.
The Eagles defensive line played pretty well overall, good enough that Trent Cole was just the third-best player despite a pretty impressive game.
Cole collected five tackles for the game. He also had a sack, fumble recovery, pass defended and quarterback hit.
It was a throwback game for the once-great Eagles defensive end.
The Eagles defense held Bengals quarterback Andy Dalton to under a 50 percent completion rate, fewer than 10 yards per completion and just a 74.2 passer rating.
But Dalton threw one touchdown against no interceptions, and it was a big one. His five-yard pass to A.J. Green in the fourth quarter capped off a 24-point scoring output in a four-minute span.
Todd Bowles' unit has now gone eight straight games without an interception.
There existed the slimmest of all possibilities that the Eagles would bring back head coach Andy Reid if the franchise ended the season with a four-game winning streak, like it did in 2011.
But the Eagles fell flat on their face when it mattered the most against the Bengals. Reid's fate has been all but sealed. He will not coach for the Eagles in 2013.