The Frenchman's departure will leave a huge void on the pitch, where he's long been considered among the best right-backs in world football; and in the locker room, where his seven years at the club made him a veteran voice in a perpetually young Arsenal side.
As posted on the defender's Instagram account, Sagna looked back on his Arsenal tenure fondly, saying:
... i just wanna thank the whole Arsenal fc... a family to me , a club who taught me so much in 7years...a club who change the kid i was to the man i am today...
A club statement reported by BBC echoed Sagna's sentimentality:
"Everyone at Arsenal Football Club would like to thank Bacary for his fantastic contribution and wish him all the best for the future."
If he wanted a simple money grab, he could've opted to return to Ligue 1 and sign a big-money deal, as the uber-wealthy duo of PSG and Monaco were both interested in acquiring the veteran full-back, via the Express.
However, his inter-league move elucidates his Premier League title ambition, and signifies his unmitigated desire to win at the highest level—a desire he was unable to indulge at the Emirates.
Since joining the Gunners in 2007, the 31-year-old won just one trophy—the 2014 FA Cup. He could've decided to remain at the Emirates, where he would've reprised his role as starting right-back for the next few seasons, but chose to accept a reduced role for a better shot at a league title.
He'll be Pablo Zabaleta's backup, but with the amount of competitions City are involved in on a regular basis, Sagna should still see significant pitch time.
It's never easy to lose a player as influential as the Frenchman, and Arsene Wenger will be expected to dip into the transfer window to find Sagna's replacement.
He's leaving big shoes to fill, as even in his final season at the club, Arsenal's No. 3 was a massive part of a Gunners defense that ranked as the league's fourth-best in terms of goals conceded.
Sagna ranked in the top-five for tackles, interceptions and aerial duels won per game amongst Arsenal players who appeared in more than three league matches, according to statistical database Whoscored.
Should Carl Jenkinson Start for Arsenal?
The only natural right-back now on the Gunners' books is Carl Jenkinson. While Jenkinson clearly gives his all for the team when called upon, there are better options out there as the 22-year-old would be best utilized as a rotation player at this point in his career.
Wenger will need to dip into his transfer kitty, and with the premium that quality full-backs command, a sizeable portion of that fund may now be spent on Sagna's replacement.
This may limit what Wenger could splash on his needs for other areas in the squad.
Let's take a look at some of the players who have been tipped to succeed Sagna, as well as others who haven't been formally linked with a switch, but could fit nevertheless.
With many of world football's elite full-backs already playing at top clubs, Wenger's options are limited.
That's not to say there isn't quality available and one rumor that is sure to ramp up in the coming weeks is Arsenal's purported move for Ivory Coast international Serge Aurier.
The Toulouse man was the best player on the pitch in his team's World Cup opener against Japan, assisting on both goals in a 2-1 Ivory Coast win.
A move to the Gunners seems to interest the 21-year-old, via Sky Sports, and with the track record Wenger has of developing Ligue 1 talent, it could be the perfect match.
Nick Ames of the Guardian (h/t FIFA), quotes Didier Drogba saying that his compatriot is the best right-back in France and the stats certainly support that claim.
Aurier's six goals were the most of any Ligue 1 full-back and tied him with Saint-Etienne's Loic Perrin for most of any Ligue 1 defender. His six assists were tied for sixth in all of Ligue 1 and second amongst full-backs, behind just Lyon's Henri Bedimo, who finished with a ridiculous 10 helpers.
According to positional data from Whoscored, Aurier made the vast majority of his appearances as a right-wing back in Toulouse's 3-5-2, shouldering huge attacking and defensive onus. An switch to Arsenal may mean Aurier would be deployed in a more traditional right-back role, but would still be given lease to get forward as the 4-2-3-1 Wenger employs is heavily reliant on overlapping full-back play.
Valued at just over £5.7 million by Transfermarkt, Aurier has started his World Cup 2014 in sensational form, and will be a hot property following the conclusion of Ivory Coast's campaign.
His stock is way up, and should Toulouse sell their prized defender, the club could—and should—demand at least double that £5.7 million valuation.
Aside from Aurier, who has to labelled as the top right-back target, there are other suitable players to be had.
Sebastien Corchia has long been linked with a move to the Emirates, per Wayne Veysey at Goal, and after a January transfer to Lille fell through, Wenger could cut a bargain for the FC Sochaux standout, who has just a year left on his contract.
Newcastle's Mathieu Debuchy was arguably the league's best right-back last season and has to like the idea of Champions League football. He wouldn't come cheap, but he was excellent in the 2013-14 term and has Premier League experience.
Matteo Darmian and Ignazio Abate are two of Serie A's top right-backs and despite the lack of Italian presence in North London, could fit.
Darmian was excellent for Torino last season and could see an extended run this World Cup. The former AC Milan man started in Italy's 2-1 win over England and with Mattia De Sciglio battling injury, he'll have the platform to showcase his quality.
Abate, like much of the Milan squad, was disappointing in their 2013-14 seventh-place Serie A finish, but has been one of the better right-backs in world football over the past few years.
He has fantastic speed and uses it to great effect, closing down attackers and getting forward. Negotiating with Milan wouldn't be easy as the club's brass would try to squeeze every cent out of Abate, but he's a solid, experienced footballer.
Which right-back do you want Wenger to sign?
There's a strong German corps in place at the Emirates and the Bundesliga boasts a number of quality right-backs that Wenger could look to add.
Borussia Monchengladbach's Tony Jantschke split time between centre-back and right-back duties last season and was a pivotal cog in Gladbach's formidable defense.
He's strong in the air and likes to put in a tackle, but his poor crossing limits his efficacy going forward. He's a stout defender and his presence would allow Arsenal's right-wingers to focus on creating goalscoring opportunities.
Oliver Sorg was Freiburg's standout defender last season, and with the club struggling to compete in the Bundesliga, Sorg would get top flight security and competition with Arsenal.
His averages of 3 tackles per game and 2.2 interceptions per game are superb and his 1.6 dribbles per game illustrate his will to support the attack.
Eintracht Frankfurt's Sebastian Jung is the most potent attacker of the three. He racked up seven assists in 30 matches last season and is a well-disciplined defender.
He isn't as prolific a tackler as either Jantschke or Sorg, but his ability to overlap makes him a nice fit for the 4-2-3-1.
Statistics are courtesy of Whoscored unless otherwise noted.