It's certainly a thought most supporters of the Blaugrana won't want to dwell on for long. However, that day will arrive sooner than you might think.
Almost nine years have passed since he made his first-team debut, replacing Deco in a match against Espanyol on October 16, 2004.
Another nine years would take Messi to 35 years old. Given the battering that he takes week in and week out on the football pitch, you could be forgiven for thinking that it's unlikely he will still be playing at the very top level by that time.
The genius we have enjoyed for what seems like the shortest time will soon become part of football folklore—a fleeting dalliance on the path to greatness, joining such luminaries as Pele, Beckenbauer, Cruyff, Platini and Maradona.
At the present time, Lionel Messi is Barcelona. His influence pervades throughout the club, not just on the pitch. He is their best player by a distance and their most high-profile ambassador, and Barca will do well to use him more sparingly.
Already this season we have seen a more pragmatic view taken on the Argentine by new manager Gerardo Martino, and you have to question why no one other than he has identified the need for the same before now.
The ongoing burden of carrying this team is a load that will, eventually, take its toll. Perhaps we are already beginning to see the early signs, given Messi's recent inability to complete a full 90 minutes (Dermot Corrigan, ESPN).
So how do Barca cater for the inevitable? What is the impact of a "Messi-less" Barca?
Over the course of the last few years, his absence from the side has seen little difference results-wise.
According to FCBarcelona.com, in the 14 games that Messi missed between 2010-2013, Barca remained unbeaten, winning 13 and drawing just one.
Yet there is a glaring omission from that list—one that club president Sandro Rosell's propaganda machine has conveniently neglected to mention since.
La Liga, in general terms, is not providing Barca with anything like the test they require on an ongoing basis, in order to measure just how much they can progress sans Messi.
For example, just a couple of weeks ago, despite their best efforts in a hard-fought game, Malaga were still no match for their more illustrious opponents.
It is in the Clasicos, or the latter stages of the Champions League, where Messi's absence will surely be felt more keenly in the future.
That is where the lack of his genuine creativity and unbelievable knack for the sublime, ridiculous or downright magnificent is writ large into the psyche of all Barca fans.
Despite this, there is no real need for doom and gloom. Barcelona have much to look forward to. Their evolution over the years has consistently seen them adequately compensate for the loss or eventual sale of their talismen.
Aside from the occasional foray into the transfer market for a "galactico" style signing (Ronaldinho/Neymar), La Masia's production line is still churning out super home-grown talent with alarming regularity.
The two Sergis—Roberto and Samper—Alejandro Grimaldo, Gerard Deulofeu and Jean-Marie Dongou are just a few stars of the future. There are others at the slightly younger age groups.
Dongou now finds himself training with the first team, and as far back as 2011 Sky Sports' Graham Hunter told totalBarca that he thought the Cameroonian was the best 16 year old he had ever seen:
I saw Messi at sixteen but Dongou from Cameroon is the best footballer at sixteen I’ve ever seen in my life.
Dongou, small and Mike Tyson-powerful, is without question the most awe-inspiring player I have ever seen at his age. When I witnessed Messi at the same stage of his development, it later transpired that he was a little bit sulky at being forced to play on the wing in the game I went to watch and, consequently, I came away unimpressed.
But having watched Dongou for several months – aged both 15 and 16 – I have to warn the next couple of generations of defenders that they should start having nightmares now just to get in practice.
The Cameroon-born striker has the low centre of gravity that made Romario so hard to contain, the ability to calculate an acute angle of snooker champion proportions, and an explosive eruption of pace reminiscent of a cobra’s head darting forward to attack.
It's debatable that any player will ever have the same impact that Messi has had on football per se, let alone FC Barcelona.
Someone that can score 92 goals in a calendar year, score in 21 consecutive games, win four consecutive Ballon d'Ors and provide something close to a man-of-the-match contribution each time he steps onto the pitch, is clearly a once-in-a-lifetime talent.
However the oft-used football cliche "no one is bigger than the club" still rings true.
Perhaps Barca should really be concerning themselves with a perceived lack of a "Plan B" in the interim. Their over-reliance on the player in the short and medium term needs to be addressed.
Football will be the poorer when Messi hangs up his boots, but Barcelona can be enriched if they execute their plans correctly.