Until now, Nacho Monreal had been an ever-present in the Premier League, starting every single game, but the Spaniard picked up an ankle injury in the tumultuous 3-3 draw with Liverpool last Friday and will be sidelined for around 10 days.

It means the Arsenal manager will likely recall Shkodran Mustafi to the starting lineup alongside Laurent Koscielny in a back four, and once more it will be interesting to see who he chooses at left-back.

In the last three fixtures he's played 20-year-old midfielder Ainsley Maitland-Niles, who has done as well as can be expected. He's being asked to do a difficult job in an unfamiliar position, and although his positional inexperience was something Liverpool successfully exploited, he's acquitted himself with some distinction and added to his burgeoning reputation as an exciting young talent at the club.

And yet there have to be questions as to why he's playing there at all. Wenger has Bosnian international Sead Kolasinac, a much vaunted summer arrival from Schalke, warming the bench, while Monreal could easily have been used in his best position with one of Mustafi, Calum Chambers, Rob Holding or even Per Mertesacker used at centre-half.

There is only so long you can get away with playing someone out of position and using Maitland-Niles as a full-back can only be a short-term fix. It's a sticking plaster decision that hasn't really done much to make Arsenal more defensively secure and it needs to be addressed.

The Frenchman has, in recent weeks, shifted back to his preferred defensive set-up: a back four. It shouldn't be overlooked how radical a change it was for him to move to a back three last season, and it was necessary to break his team out of a terrible slump in form.

What's odd about is that it came so soon after one of the team's most assured defensive performances with the three centre-halves, their 2-0 win over Spurs in the North London derby. While Wenger himself never appeared 100 percent convinced with it, it looked as if the players were and his reversion to the back four and the use of Maitland-Niles as a full-back has probably created some general confusion again.

However he decides to set up his defence for the game at Selhurst Park, he also has to get his team firing again up front. Goals have been in relatively short supply this season -- the Gunners are averaging just under 1.8 goals per game, down from 2.2 goals per game at this stage in the last campaign -- and the absence of Olivier Giroud is a blow in that regard.

At this time of the year, with so much football to play, squad depth is critical, and there doesn't appear to be a great deal of that from an attacking point of view. In 2017, Giroud has scored 10 goals after the 80th minute, a quality that's not apparent in the likes of Danny Welbeck, who struggles when introduced from the bench, or Theo Walcott -- a player so far on the fringes that he has just over 45 minutes of Premier League action to his name this season and not one start in this competition.

Alexandre Lacazette is the club's record signing, but just two of his eight goals this season have come away from home, and he also has to cope with the responsibility of his price-tag on top of the new challenge of playing through a period when he'd normally be enjoying a winter break.

The physical demands of that on players who are not used to it shouldn't be overlooked. He's a player who Wenger has nursed through the opening six months of his Arsenal career, managing his fitness carefully, building him up to the rigors of life in England. Now, he's might have to play three games in seven days on top of the 90 minutes against Liverpool last Friday.

It's going to be a real test for him, and for the team as well. The festive period can have a significant and rapid effect on the table, so having seen those around them win already, the pressure is well and truly on Arsenal to take all three points against Palace.

Away from home they've been unconvincing, and Thursday night would be a very good time to start putting things right.