And it doesn't take a rocket scientist to figure out that the two-time Super Bowl MVP struggled.
"Obviously, I didn't play well last night," Manning said Monday. "And there is no hiding it. I didn't play my best football. I got to play better. I think that is all I am worried about."
After orchestrating 23 regular-season wins in which the Giants trailed or were tied in the fourth quarter, Manning's struggles in the final quarter against Pittsburgh were uncharacteristic and stunning.
He completed just 1-of-5 passes for a yard and was sacked twice as the Steelers erased a 20-10 deficit in the fourth quarter, scoring 14 unanswered points.
But Manning also has not played up to his high standards in the past three games. He has just one touchdown to four interceptions in that span.
Since passing for 337 yards against the Redskins, he has 317 yards in the past two games combined.
Coach Tom Coughlin and Manning were asked if the team is in a slump.
"This is not baseball," Coughlin said.
The Giants went three-and-out on all three of their fourth-quarter drives and managed a total of negative-8 yards in that quarter.
"Whatever you want to call it, we are not playing good football," Manning said. "And that is what it is. No one is going to come in and help us out."
"I got to make some throws and just get back to having great practices and not leaving any throws out there," he added.
The defense also was victimized by running back Isaac Redman (147 yards and one touchdown) and could not get a critical stop in the fourth quarter.
"It's frustrating," defensive end Justin Tuck said. "The phrase that's going around this locker room is 'the pain of regret' because we had opportunities to win that game. Give the Steelers a lot of credit. They came in and played a physical brand of football."
For the Giants, physicality and efficiency on the offensive side of the ball have been lacking, and the passing game's fourth-quarter magic act has vanished.
Manning's only touchdown in his past three games has been his 77-yard bomb to Victor Cruz with 1:13 remaining to outlast Washington, 27-23. It was after that play, in which Manning and Cruz said they saw something they could exploit in the Redskins' defense, that Redskins defensive back DeAngelo Hall said Manning wasn't a "rocket scientist" who suddenly figured something out, but rather a breakdown.
Opposing defenses apparently have figured out a blueprint to slow down Manning and his playmaking duo of Cruz and Hakeem Nicks.
Cruz and Nicks have a combined 24 catches in their past three games. They are capable of having that many receptions in just one game.
Cruz said there have been some communication issues lately. Coughlin could not answer why that is after Cruz and Manning had hooked up for seven touchdowns in their first seven games.
"That's a good question," Coughlin said. "I don't think much has changed. I think for some reason we are not clicking like we were. Two weeks ago you had the number one big-play team in the league."
Now, the Giants can barely convert a third down. In the past two games, they have converted only five of 25 third downs.
"A lot of people are doing different things to us coverage-wise," tight end Martellus Bennett said. "We got to do a better job of getting open. Guys haven't been blitzing us as much, dropping more in coverage than rushing. Everybody has been game planning for us differently than they had been playing against other people previously on tape."
"We haven't been able to hit as many deep balls," Bennett added later. "There have been more guys in coverage. It seems like there are a lot of opposite-colored jerseys out there. We saw a lot more of Cover-4 and different forms of Cover-3. They had a lot of man lock on the outside in a Cover-4 type form."
Manning said it should not matter what defenses throw at them.
"I have gone through stretches where I haven't played great football and been able to bounce back and start playing better," said Manning, who has just two touchdowns in the past four games since throwing three Oct. 7 against Cleveland.
"You know, I didn't play great versus Dallas and we won," he added later. "(The Giants) can win but obviously offensively we have to do our part. We got to play better offensively to help out this team."