Spain had gone ahead through a Mikel Oyarzabal penalty when Saliba crumpled without contact just past the half-hour mark, immediately reaching for his lower back before being replaced by Crystal Palace's Maxence Lacroix.
The 25-year-old had nursed the same issue since it flared up during Arsenal's Champions League final defeat, managing it through France's run to the last four before it finally forced him off in Dallas.
French outlet L'Equipe had already flagged surgery as a possibility before the setback against Spain, and that option is now expected to feature prominently in discussions between Arsenal's medical staff and the player once his summer duty with France concludes.
Any operation would typically carry a lengthy recovery period, complicating Arsenal's plans for the start of the new season. According to the Daily Mail, Saliba told team-mates on the pitch that his back had gone as he went down, underlining the severity of the setback in what had otherwise been a strong tournament for the defender.
Statistically, Saliba had been in outstanding form throughout the injury-hit stretch. Data from Opta shows his defensive unit conceded just one non-penalty goal across his previous 13 appearances for club and country, spanning 1,155 minutes, with that solitary strike coming in stoppage time of a France win over Senegal.
Last term brought a heavy workload for Saliba, who featured 50 times across all competitions as the Gunners ended a 22-year Premier League title drought and went on to lose the Champions League final to Paris Saint-Germain, a match in which he played the full 120 minutes.
Discussing the injury during the World Cup group stage, before it resurfaced against Spain, Saliba said: "I've had some minor niggles for several months.
"I've been gritting my teeth because there was the Champions League and the Premier League.
"The World Cup comes round only once every four years, so you've got to grit your teeth."
He sat out the final group fixture against Norway as a precaution but was recalled for the knockout rounds, starting every game through to his exit against Spain. With France's tournament now firmly over, attention turns to how quickly - and how safely - Saliba can be built back up ahead of Arsenal's Premier League title defence, which begins next month.