Arsenal manager Mikel Arteta has suggested he regrets taking the "hurtful" decision to leave William Saliba out of his Europa League squad.
The Gunners host Dundalk on Thursday with just two recognised central defenders available to Arteta.
Arsenal have eight on their books but, due to a mixture of injury and unavailability, Shkodran Mustafi and Gabriel Magalhaes are the only options.
David Luiz limped out of Sunday's Premier League defeat to Leicester to join Rob Holding, Calum Chambers and Pablo Mari in the treatment room.
Mesut Ozil's axing from Arsenal's Europa League squad may have made all the headlines but two more defenders, Sokratis Papastathopoulos and Saliba, were also excluded.
Saliba has yet to make his Arsenal debut after returning to the club this summer.
The 19-year-old joined Arsenal from St Etienne last summer for a reported £27million – but the agreement also meant he would remain with the French side for the 2019-20 campaign.
Plans to loan him out to a Sky Bet Championship club this season were eventually shelved but he has taken time to settle in north London.
Now Arteta would like to have him available against Dundalk – but cannot call on him after he did not make the cut.
"I feel really bad for William Saliba," he said. "Because we had so many central defenders, we decided to leave him out of the squad which was really hurtful for me to do.
"I was hoping that Pablo would be back in two weeks but he had a setback and then we don't have Pablo and we don't have William when he's fit and available to play.
"But when you make those decisions, you can't always think about every possible outcome."
Asked if Saliba was making progress after a tentative start to life at Arsenal, Arteta added: "Yes, he's in a much better place.
"He's feeling a lot more confident around the place, his language is improving and he's starting to understand much better what we're doing physically.
"He's played a few games with the under-23s which he needed because he didn't play any football in the last seven or eight months. Things are progressively getting better and better."
