The Japan international has impressed in his three games since joining the Gunners from Bologna on transfer deadline day.
Speaking in the match-day programme for Sunday's win over Tottenham, the 22-year-old defender said: "For me Arsenal is one of the biggest clubs in the world, so there is no reason at all to say no to them. It's as simple as that – when I had the offer, I just wanted to be part of Arsenal.
"I wanted to play at a higher level, so I am so grateful to be given this chance by Arsenal. Just could not believe it when I found out.
"I was very excited as soon as I knew. I didn't need to call anyone or speak to anyone for advice. It was an easy choice for me, because, well because it's Arsenal.
"I didn't even have time to speak to the manager before signing the contract, and I didn't need to. I wanted to come."
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