Merino was signed from Real Sociedad as a “win now” move to bolster the team’s Premier League chances for this term.
However, the 28-year-old is out with a shoulder injury he picked up in his first training session.
Manager Mikel Arteta spoke about the incident, stating at the weekend: “Well, he had the incident and he carried on playing - and training.
“And at some point we were looking at him and thought he did not look right. That shows you how much he wanted it and he did not want to accept that was happening in that moment.
“But actually, that is what happened and now let’s use the time that we have, right now, to make sure he is familiar with everything that we are doing, how we live and what we expect of him, about our team dynamics.
“He will be in every team meeting as well. We will use the time in the best possible way and in a few weeks he will be ready. It is very disappointing because you can feel the pain of the player as well.
“We fought so hard for three months to get the player over the line. The club made a huge effort to bring him in and on day one for that to happen, it’s a huge blow.
“But his reaction has been exceptional straight away. We signed him for many years, not for a few weeks and we need to move on. These things can happen, it is very unfortunate but this is our reality.”