Merino has fractured his foot, and Arteta claims the extent of the injury is "rare".
He added: “Let's see. It's a very rare injury, so we have to wait and see once we have the surgery. How that reacts daily.
“Obviously, monitor it throughout the week for the surgery. I know Mikel is going to do absolutely everything that he possibly can to come back as quick as possible.
“But, as well, he has to respect the healing process and the fact that, obviously, it's an injury that is quite rare.”
Merino will also hope to return before the end of the season as he aims to represent Spain at the 2026 World Cup in the United States, Mexico, and Canada.
It is a blow for a player who was revelling in his side's fine form earlier this term, telling the club's website: "Obviously, football is not only about results. It's the main thing, but when you can enjoy playing with these teammates, it's just a joy, and I'm enjoying it every time I'm on the pitch.
"I think everyone is giving their best and enjoying as well, and that's the best mix we can have, so performing and enjoying is the magic recipe, so yeah, very happy."
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