Kostas Tsimikas reportedly broke his collarbone on Saturday as he collided with Jurgen Klopp on the sideline. The Liverpool defender had to leave the pitch, of course, leaving the Reds without a proper left-back.
Andy Robertson has his own issues, after all. The Scot has been out for a while after shoulder surgery and it isn't exactly clear when he'll return.
That left Klopp with one option - Joe Gomez. Gomez is capable of playing left-back but it's certainly his third-best role; the defender is much more at home playing centre-back or right-back.
Not that you could have known that by watching him against Arsenal. Gomez entered the fray on 35 minutes and put in one of the most dominating full-back performances you'll see all season.
The highlight statistic is that Gomez won six tackles. Not only was that the most of anyone on the pitch but he also attempted six - meaning he won every single tackle. Doing that at a match-high is extraordinary, let alone doing it in 55 minutes.
Gomez was also up against arguably Arsenal's star player, too. Bukayo Saka got no real joy on that flank after Gomez arrived.
And on top of it all, the Liverpool defender registered five clearances (most for Liverpool, 2nd-most in match) and made an interception, while only Gabriel Jesus and Mohamed Salah had more than his two shots.
It was a remarkable display and one that shows just how good Gomez can be. The defender has had his ups and downs over the last few seasons but at his best, he remains one of the most complete defenders around.
After all, how many can put in top performances across the back four? Gomez can - even in games as big as this.