The Frenchman has just returned to fitness after suffering a nasty Achilles injury during the Europa League semi-final last season.
Koscielny was on the scoresheet against Chelsea on Saturday but suffered an early exit this evening after colliding with Lukaku.
While duelling for the ball, the defender hit the ground behind the Belgian, where he was then caught by the trailing leg of Lukaku.
Koscielny immediately waved for play to be stopped while holding his face on the floor – and the cameras revealed the true extent of his injury.
Lukaku’s boot clearly caused a heavy blow, as Koscielny’s head was bleeding profusely.
Medics rushed on the pitch and attempted to patch the Arsenal captain up.
But eventually, despite trying to continue, the injury forced Koscielny off the pitch.
“Tell you what that’s nasty on Koscielny….Total accident but feel for the lad,” one fan wrote on Twitter.
“Koscielny injury looks really nasty. Blood oozing out the side of his head. Hope he is okay,” one said.
“Thats probably a broken jaw for Koscielny.. very unlucky,” another wrote.
“Nasty one for Koscielny but total accident. Hope he`s ok,” one said.
“Honestly hope that’s nothing bad for Koscielny, still a very good player,” another added.
Speaking this week, Unai Emery spoke about his captain.
“In the last matches, I think we are looking at a Koscielny with a big performance like he is used to producing.
“After these injuries, it’s also necessary to take matches, to take confidence again and then physically, for him to be well enough to play with this level of performance.
“I think in the last two matches, at West Ham and against Chelsea, I think this year and after his injuries, they are the performances we want.”