For four weeks, Jackson started because Flacco was dealing with a hip injury. Now, with Flacco fully recovered, the Ravens are sticking with Jackson because the team feels the rookie first-round pick gives Baltimore the best chance of winning and reaching the playoffs.
"Every decision is based on what makes us the strongest possible team we can be. ... That's what it boils down to," Harbaugh said. "That's how we feel about this decision and we're rolling."
Flacco, 33, has been Baltimore's starter since being the No. 18 overall pick in the 2008 draft. He was named Super Bowl MVP in 2012 when he led the Ravens to the franchise's second Lombardi Trophy. His 106 victories, including playoffs, rank fifth among quarterbacks since 2008, trailing Tom Brady, Drew Brees, Ben Roethlisberger and Aaron Rodgers.
This marks the first time in Flacco's 11-year career, which spans 163 games, that he will serve as a backup.
"Obviously [I'm] disappointed that I can't be part of this team in the same capacity I have been for a long time," Flacco said. "... I can't say I was surprised."
One of the most durable quarterbacks in NFL history, Flacco had not missed a game in nine of his previous 10 seasons. He injured his right hip on the opening drive of a Nov. 4 game against the Pittsburgh Steelers and finished the game.
Flacco, who is 4-5 as the starter this year, wanted to play through the injury, but doctors told him that he could injure himself further. He wasn't cleared to play again until last week.
"Listen, it's out of my hands," Flacco said. "I got hurt; they drafted Lamar in the first round. At some point, something was going to happen between the two of us. Who knows what that was going to be.
"This is just what it is at this point. I've obviously had five weeks to think about it and prepare myself for this situation and the possibility of it. ... I'm going to do my best to handle it the right way."
The Ravens moved up to the bottom of the first round to select Jackson with the No. 32 overall pick. He was used in specialty packages throughout the first half of the season before Flacco's injury pushed Jackson into the starting role.
Jackson went 3-1 in Flacco's absence and helped get the Ravens back in the playoff hunt. He has provided an emotional spark and brought a run-dominated offensive identity to Baltimore (7-6), which currently holds the No. 6 seed in the AFC and sits a half-game back of the Steelers in the AFC North race.
Jackson has been more of a threat with his legs than his arm. His 336 yards rushing are the most by a quarterback in his first four starts in the Super Bowl era.
Jackson received the news from Harbaugh on Tuesday, and he essentially welcomed it with a shrug.
"I pretty much didn't really have a reaction to when he was telling me," Jackson said. "All it made me do is -- I know I have to focus on everything a lot more, just bettering myself and trying to join with everyone around me even more. That's about it."
Flacco's demotion likely means he's entering his final games with the Ravens. Since winning the Super Bowl, Flacco has failed to live up to expectations as one of the highest-paid quarterbacks in the NFL, throwing 116 touchdowns and 82 interceptions in 85 games, including playoffs.
His 82.7 passer rating since 2013 is 37th in the league over that span, ranking behind Tyrod Taylor, Josh McCown and Colin Kaepernick. The Ravens can create $10.5 million in cap space by releasing or trading Flacco in March, or free up $18.5 million by doing so after June 1.
"There's plenty of time for that," Flacco said when asked about his future. "Obviously, I think it's only natural for people to kind of talk to you about that and all of that stuff. But there's plenty of time for that. There's plenty of months in the NFL offseason to think about what it's going to mean and all that. But for now, I'm excited for what I can do and how I can help the team out."
So does Jackson feel this is his team now?
"This is our team, all of us together," Jackson said. "I don't go out there and block. I don't go out there and catch the ball. I don't make tackles. I just do my part. It's all our team."