In Manning's last 13 games including playoffs he has put up the below stat line:
266 / 496 (53.7% completions) with 13 touchdowns, 16 interceptions, and 2,904 yards (223 per game and only 5.9 per attempt). That's a 66.6 QB rating.
In that time, it's clear to see that he lacks zip on his passes, balls are floating, he's frustrated, and he's making poor decisions.
This year he is 31st in QB Rating. He's 22nd in completion percentage. 17th in yards. 26th in TDs. 31st in yards per attempt. All while leading the league in interceptions.
He's been a significantly less efficient passer than Colin Kaepernick this year. Colin bloody Kaepernick. And he doesn't even have the mitigating factor of being able to run.
Football Outsiders has a metric they call defense-adjusted yards above replacement and another they call defense-adjusted value above average. It's intended to measure a player's performance relative to the rest of the league and adjusted for the strength of the defenses he's going against. Manning ranks 33rd out of 33 qualifying quarterbacks this year in both metrics.
I wasn't using hyperbole when I compared him to Rex Grossman. He's been that bad.
Born and bred a cheesehead