In the midst of his best game of the season, Flacco took a gruesome shot to the head late in the first half from linebacker Kiko Alonso after sliding to the ground on a run. Flacco's helmet immediately flew off of his head, and the Super Bowl-winning quarterback went to the locker room with a glassy-eyed look and blood coming from his left ear.
Alonso's hit on Flacco drew outrage from the Ravens as well as an unsportsmanlike conduct penalty. Ravens coach John Harbaugh came onto the field to yell at Alonso in a game that featured several fights the rest of the way.
Flacco was ruled out for the game during halftime. It is the first reported concussion in Flacco's 10-year career.
"Joe had a concussion and a cut ear," Harbaugh said. "They were stitching that [ear] up without any anesthesia, so he's a tough dude. That's as much as I know about it."
When asked postgame whether he thought he'd be suspended, Alonso said: "That's out of my hands, man. ... It's a bang-bang play, and I hope he's all right. I truly do."
Alonso said there was "no way" he could have avoided the hit.
"When a guy slides, the target is very small. I just think it [Flacco's slide] was a second late, which is why I hit him, to be honest with you," Alonso said. "At first I was anticipating I thought he was going to slide. And then it got to a point where I was like, 'I got to him,' because he slid too late."
Harbaugh declined to comment on whether Alonso should've been ejected.
"It was penalized correctly, I would say," Harbaugh said.
Several Ravens players also expressed their frustration with the hard hit.
"If you mess with one of us, you got to mess with all of us. We went out there and gave everything we had for Joe, the team, the coaches and Baltimore," Ravens defensive tackle Brandon Williams said. "We had to let everybody know that you can't just mess with one of us and know expect to get hit 53 more times."
Ravens offensive lineman Ronnie Stanley said Alonso should have been ejected.
"For a hit like that, I think that's the right call for that," Stanley said.
NFL rules allow any player, including quarterbacks, to end a play without contact by declaring himself down. Thursday night, Flacco attempted to do so by sliding feet first. In those situations, according to the NFL rulebook, the ball is dead "the instant he touches the ground with anything other than his hands or feet."
The rule instructs defenders to "treat a sliding runner as they would a runner who is down by contact" and "pull up when a runner begins a feet-first slide." It allows flexibility for a defender who might not be able to avoid contact but still prohibits the kind of contact Alonso initiated on Flacco.
"If a defender has already committed himself," the rule states, "and the contact is unavoidable, it is not a foul unless the defender makes forcible contact into the head or neck area of the runner with the helmet, shoulder, or forearm, or commits some other act that is unnecessary roughness."
That's undoubtedly what referee John Parry saw when he penalized Alonso for unnecessary roughness. The rulebook provides referees with the option to eject players in situations in which the contact is flagrant, but Parry elected not to. The NFL defines "flagrant" as "extremely objectionable, conspicuous, unnecessary, avoidable, or gratuitous."
Al Riveron, head of officiating for the NFL, told CBS that the league would not comment on the play Thursday night. The league rarely comments right away, instead choosing to watch the film before talking about a ruling.
"I thought it was a dirty play, personally," Ravens wide receiver Jeremy Maclin said. "I don't think Kiko's a dirty player, but I thought that particular play was dirty."
In replacing Flacco, Ryan Mallett had to complete only 3-of-7 passes for 20 yards and a touchdown because of the Ravens' sizable lead. Baltimore has played just six games without Flacco since 2008, going 2-4 in his absence.
"Hopefully Joe can get himself ready to play," Maclin said. "But if not, we have to ride with Mallett. It's the next man up."
ESPN's Kevin Seifert contributed to this report.