Arsenal have had a nightmare with the fitness of their starting full backs over the course of the previous season. In fact, I would go as far as to say it was one of the determining factors behind missing out on the Champions League places.
For comparison, Chelsea, initially touted as title challengers lost both Ben Chilwell and Reece James for an extended period and watched their season peter out. Arsenal lost Takehiro Tomiyasu for effectively half of the campaign and Kieran Tierney for the entirety of the run-in.
This meant that backup options Nuno Tavares and Cedric Soares had plenty of game time. The Portuguese pair had moments worthy of praise, but the final run-in saw the worst of them both.