It would be churlish to suggest Kieran Tierney was Arsenal's man-of-the-match against Crystal Palace given he wasn't even out there for the 0-0 draw but the left-back's importance to Mikel Arteta was underlined in his absence here.
Tierney had helped set the tone for the Gunners' post-Christmas revival which saw them end a run of seven winless Premier League games with three consecutive victories and an extra-time FA Cup success against Newcastle. The decision to play Tierney for the entire 120 minutes of that third-round tie when several other regulars were given at least part of the evening off was both an understandable desire to keep an in-form player in the team but also an unnecessary risk viewed in the context of what followed.
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Tierney was unavailable against Palace due to what the club initially stated was unspecified "muscle tightness", code perhaps for cumulative fatigue given this was only the second Premier League game he hadn't started all season. But it later emerged Tierney had been forced out of the game just a few hours before kick-off and will undergo an MRI scan on Friday to determine the extent of the problem.
"We'll probably know more tomorrow after testing him, having the evolution of the injury, and how long he is going to be out for," said Arteta.
"Certainly, it was a big disappointment to lose him. It is a challenge when you make the gameplan for the day and within a few hours just before the game, he has to pull out. Kieran's progression in the last few months has been incredible and now it is for other players to step in."
No Arsenal player has created more chances from open play this season than Tierney's 20 and they desperately missed him here. Ainsley Maitland-Niles has been tried as a left-back from time-to-time -- most significantly perhaps in Unai Emery's first game in charge against Manchester City in Aug. 2018 -- and as a left wing-back when Arteta favoured a 3-4-3 formation during Project Restart but it is not a position he is naturally suited to. The 23-year-old struggled from the outset defensively and failed to provide even a vague approximation of Tierney's attacking threat, registering just one cross in the 65 minutes before he was substituted.
Something similar could be said of Hector Bellerin on the opposite flank, who managed to work himself into more dangerous positions but frequently made the wrong decision, most memorably taking the ball off Alexandre Lacazette's toe as he prepared to shoot from Bukayo Saka's cross. Bellerin's form has been underwhelming for some time now, and Cedric Soares must be wondering what he needs to do to get a run in this team. But the reliance on Tierney at left-back is more profound following the decision to allow Sead Kolasinac to rejoin Schalke on loan until the end of the season.
Along with Maitland-Niles, who still harbours hopes of establishing himself as the midfielder he always planned to be, Saka is the next-best understudy for Tierney and yet deploying him there robs the Gunners of one of the brightest attacking talents in more advanced areas.
Saka and Emile Smith Rowe showed up their more celebrated teammates Willian and Nicolas Pepe against Newcastle but they struggled to break down a typically well-drilled Palace side on this occasion. Smith Rowe has thrived of late but to ask a 20-year-old to repeatedly deliver with just a handful of top-flight games under his belt -- this was just his fifth Premier League start -- is perhaps a measure of the transition period Arsenal are in the midst of.
Width is often the key in these situations but with Maitland-Niles and Bellerin unable to make inroads, Arsenal found themselves disappearing down cul-de-sacs and lacking creativity reminiscent of the most troubling periods during Arteta's tenure.
This was the end of a three-game winning run -- four if you count that extra-time FA Cup victory -- and so there is a sense of momentum being checked. Arsenal haven't beaten Palace since Arsene Wenger left the club in the summer of 2018, a run now stretching back four draws and one defeat, and the lack of consistency in performance which contributed to the Frenchman's departure continues to linger as a leading issue.
The sheer volume of games is catching up with a lot of teams and Arteta believes Arsenal are no different.
"We created a lot of situations against the low block but lacked the final quality," said the Spaniard. "We probably didn't do enough to win the game. We are playing a lot of games and you can see fatigue is paying a price, many players aren't fresh.
"We had a period that was fatigue-related where we kept giving the ball away. We lacked the quality in the final third to win the game."
Arsenal's more attacking players must take responsibility but in the past, they have been bailed out by Tierney bombing forward from deep. Arteta will be praying his marauding left-back isn't out for long.