Calum Chambers is facing a lengthy lay-off with a long term knee injury.
Chambers initially hoped it was more of a tweak but the prognosis is a major worry for Arsenal who stand to lose another defender for an extended period of time.
Gunners defender Chambers has been outstanding this season, playing both at full back and centre half while also showing an attitude which has already impressed new boss Mikel Arteta.
It is another slice of bad luck for Arsenal who lost Rob Holding and Hector Bellerin last season to long term knee injuries and are still fighting their way back to full fitness.
Greek defender Sokratis, Sead Kolasinac and Kieran Tierney have all been missing this season and that has led to Arsenal looking at the January window for potential reinforcements.
As Mirror Sport reported on Monday , they are keeping close tabs on Red Bull Leipzig defender Dayot Upamecano who could be allowed to leave next month with his contract running down.
France Under-21 centre half Upamecano, 21, who has 18 months left on his current deal, is attracting interest from Premier League clubs with Arsenal among the big admirers.
Arsenal have signed William Saliba from St Etienne – he is currently loaned back to the French club – and could even enquire about him joining up early.
But defence is one area where Arteta definitely wants to strengthen as David Luiz and Shkodran Mustafi are now likely to be left holding the fort against Manchester United until others return.
Arsenal spent big last summer and gambled on finishing on the top four but are likely to sanction funds especially as they are likely to lose players and there will be ins and outs.
Pierre-Emerick Aubameyang is likely to leave next summer if Arsenal do not get a place in the top four while Granit Xhaka is wanted by Hertha Berlin in January.
Arteta is ready to start a reshuffle as he looks to improve the squad with Luiz admitting that the new manager’s methods will take time to bed in after Sunday’s defeat to Chelsea but insisted the coach is “world class.”
Luiz said: “He’s trying to make a new philosophy. I think everybody saw that in the first half, we dominated the game and we should have scored the second goal. It would have been a different game.
“After that the second half was very nervous and both teams were fighting till the end. In the process they are in front of us, they had a pre-season together with the same philosophy and coach.
“It’s up to us to improve ourselves, we need to get some points, some results. We need to understand the stage of the season we’re in now. We have to take the positives from the Chelsea game and improve the negative things.
“We have to try to improve ourselves as soon as possible because this club deserves to shine again. They did the right choice with Arteta, he will be one of the best coaches in the world for sure because he knows football, he understands football.
“He’s trying to improve our team with some field training. We’re going to try to do our best. The first half everybody saw an amazing thing. The second half Chelsea were better than us.”