Arsenal chief Unai Emery feels defender Laurent Cosellini suffered a broken jaw during a defeat by Manchester United in the fourth round of Chelsea.
Koscielny got entangled with Romelu Lukaku before Belgian inadvertently caught his head during a 2-1 loss.
In the second half, eight minutes were interrupted when the French international got treatment.
The 33-year-old returned from a seven-month absence in December after his Achilles interrupted last May.
"It was maybe a break," Emery said.
"I hope not, but he goes to the hospital and we will wait."
Spaniard Sokratis Papastathopoulos also left the pitch with an ankle injury, less than a week after full-back Hector Belerin was excluded for the season with a torn frontal cruciform ligament.
"Succeeded hurt his ankle – I hope there is not much. I hope there are not two major injuries.
"We are taking very serious injuries, like Hector Belerin. Our injuries have changed the possibility of doing something different."
"We need to think about the Premier League"
The double scorer put Manchester United at the Emirates stadium, although Pierre-Emerick Aubamean replied to the hosts half an hour ago.
However, strikers of the Gunners – who hope for silver games in the Europa League – have failed to reach the level before Anthony Martial to seal the game with the remaining eight minutes.
Arsenal's manager wants his side to refocus on their four-point ambitions as they prepare to receive Cardiff City on Tuesday.
"Our game plan worked well, we had the pressure and chances, but in the second half we can take risks," Emery said.
"We need to think about the Premier League, we have won against Chelsea and we are close. We need to push the match and have another important match in the Europa League.
"We played a team at a good moment, we lost this game, but I'm happy with our work. We deserved more."