The Frenchman had been so consistently excellent across this past campaign that it wasn't a shock to see other clubs express an interest in his expiring contract, so Arsene Wenger was forced into the market to find a replacement, and he did so in Mathieu Debuchy.
Having witnessed Didier Deschamps prefer Debuchy to Sagna for international duty at the FIFA World Cup 2014, Arsenal fans could be forgiven for wondering whether they may just have upgraded the position.
Whether he's better or not will be a hotly contested debate that reveals myriad answers, but the most immediate concern for Arsenal is rejigging the balance of their defensive line.
Contrary to popular belief, the Gunners' defence was excellent last season. The blowouts were often the midfield's fault, and if not then the distance between the defensive and midfield line likely was.
Laurent Koscielny led a stubborn line, and the setup was exceptionally well balanced.
Per Mertesacker is a leader at the back and a titan in his own third, but he requires covering help due to a lack of pace. Koscielny's aggression allowed them to form a stopper-sweeper partnership and Sagna's defensive studiousness—covering and dropping back to ensure Mertesacker was never isolated—was key to clean sheets.
That freed up Kieran Gibbs to do what he does best: open his legs out and buccaneer forward. The Englishman's production, crossing and decision-making in the final third drastically improved in 2013-14.
The dynamic of a defensive line, and in particular full-back balance, is key to success. Recent history has shown that once clubs commit to two advanced, attacking full-backs, they become porous as a result. See Barcelona with and without Eric Abidal as a major case in point.
Last season's Arsenal was well balanced. Sans Sagna and with Debuchy, though, some obvious questions spring to mind.
Debuchy is a Gibbs, not a Sagna. He's a marauder, an attacking weapon, and Deschamps used the World Cup to spell out exactly what his strengths are, playing to them where possible.
The 28-year-old excels while pushing forward and stretching the pitch, feeding off an inverted winger and hitting the byline to cross. Goals from him are rare, but when they do arrive it's because he's gambled at the back post.
But with both Debuchy and Gibbs looking to step forward with every chance, it leaves potential holes all over the pitch for Arsenal—particularly around Mertesacker in space.
Gibbs' role as a full-back is well carved out; a known quantity to Wenger. The Frenchman had the choice of the transfer market—Newcastle United's €7 million signing Daryl Janmaat included—and opted for Debuchy.
It's a curious choice on paper, as it risks the very fabric of defensive semblance Arsenal were built on last season. He's a strong player, no slouch, but Arsenal look arguably weaker at the back than they did before—from a tactical and positional standpoint, at least.