Arsenal was not lucky enough during this season and they recently lost their player Mikel Merino who he joined this team just a few days ago. Mikel Merino was the player Arteta wanted to hire him and he tried to get Mikel Merino ready for the games in this season.
He left Real Sociedad and started playing for Arsenal to have the chance to play in strong teams. But in the first training session in Arsenal, he got injured and it seems Merino's going to be out for 2 months.

Injuries are like chronic big problems in Arsenal. It can come from their internal medical team problems but with cases like Merino, it's purely unlucky situation for Arsenal and it's hard to explain why it came quickly like this.
Merino scored a winning goal for Spain against Germany in Europe 2024 and contributed to his national team trophy but I am very doubtful on ability of this player as well as his chance to succeed in Arsenal and Premier League. This injury makes things worse and he will miss many matches as well as opportunities to train with new team mates. This injury will make his adaptation in Arsenal more difficult and create pressure on his come back after recover process.