The Giants didn't muster much of a challenge for the Colts, and that was never as evident as on Indianapolis' first touchdown Monday night.

Tight end Coby Fleener caught a 21-yard pass with linebacker Jacquian Williams on his back, and the ball appeared to pop out as the two hit the turf. While Tom Coughlin struggled to reach for his challenge flag and the defense was in disarray, wanting the call to be overturned, the Colts hustled up to the line of scrimmage and ran a play. They snapped the ball before Coughlin could yank the red flag from his sock -- he keeps it there to avoid impulsive challenges -- and Fleener was left uncovered for a 32-yard touchdown catch.

It was the first of four touchdown passes for Luck, who overwhelmed a thin secondary while Eli Manning and the Giants' offense could barely muster a pulse until it was far too late in a 40-24 blowout at MetLife Stadium.

Making the defeat worse was the loss of cornerback Prince Amukamara. After he left the game early, the Giants announced he has a torn right biceps, which will require season-ending surgery.

It was the third loss in a row for the Giants (3-5), who were far more productive when they weren't on the field the past two Sundays and saw their division rivals struggle. They trail the first-place Eagles by three games in the NFC East.

The NFL's vice president of officiating, Dean Blandino, wrote on Twitter that the ruling on the Fleener catch would not have been overturned had Coughlin managed to challenge in time. Blandino wrote that Fleener "had control and completed the catch before going to the ground.''

So while a prompt challenge would not have resulted in an incomplete ruling, it would have stopped the Colts from running their play while the Giants' defense was in a chaotic state.

Fleener's TD gave the Colts (6-3) a 10-0 lead with 13:38 left in the second quarter. The Giants did an admirable job against the NFL's top-ranked offense for most of the rest of the half, allowing only two field goals. And they did so without a fully functioning secondary.

Amukamara's injury meant Dominique Rodgers-Cromartie had to play more than he or the Giants had hoped. Rodgers-Cromartie was listed as questionable and was rotating in and out before the injury. Eventually, the Giants settled in with Rodgers-Cromartie and Zack Bowman as the outside corners and Jayron Hosley in the slot. They put decent pressure on Luck, but had only one sack.

The Giants' offense, however, was horrible. Challenged by the general manager to be more aggressive and by the coach to play "above the Xs and Os,'' they couldn't do either. The Giants did take several more chances with passes down the field than usual, but they came up empty.

Against a team that allowed 51 points to the Steelers a week ago, the Giants never even reached the red zone in the first half. They left the field to a cascade of boos, trailing 16-3.

Not even Michael Strahan's Hall of Fame ring ceremony at halftime could change the momentum. Indy literally plucked a touchdown away from the Giants on the opening drive of the second half when Luck threw a pass into the end zone that Rodgers-Cromartie appeared to intercept. Before he could come down with the ball, T.Y. Hilton pulled it away from him for a 31-yard reception to make it 23-3.

The Giants' offense finally found a pulse with long receptions by Odell Beckham Jr. and Andre Williams that featured strong runs after the catch -- above the Xs and Os, if you will -- that set up Williams' 1-yard touchdown run to close to 23-10 with 9:48 left in the third. But the Giants' deficiencies in the secondary bit them as Luck hit Reggie Wayne for an out pass that Hosley overextended on. Wayne ran most of the 40 yards uncovered for a 30-10 lead.

Manning fumbled on a sack deep in Giants territory, and Sergio Brown returned it to the 4 to set up a third Colts touchdown in a span of 7:21.

Notes & quotes: Rookie guard Weston Richburg was carted off in the third quarter with a right ankle injury . . . The Giants are 7-4 after the bye for Coughlin.