The 28-year-old had already established himself as a goalscoring midfielder for Real Sociedad after a failed stint at Newcastle United, while also netting his country's extra-time winner against Germany in the Euro 2024 quarter-finals.
Arsenal paid £30m to bring Merino to the club from Real Sociedad, but before the Spaniard had the chance to show what he was capable of on the pitch, he sustained a shoulder fracture in a bizarre incident.
Merino was thought to have been engaged in a tussle with Gabriel Magalhaes, who subsequently landed on the midfielder and caused his serious injury, which put him out of action for a few weeks.
However, the former Newcastle lynchpin recovered quicker than expected, and he made his belated debut for Arsenal off the bench in Tuesday's 2-0 Champions League victory against Paris Saint-Germain.
Merino looking on "the bright side" of bizarre injury
Merino will now be in line to make his Premier League debut for Arsenal in Saturday's home clash with Southampton, and speaking to the Evening Standard after Tuesday's win, he admitted that he had never seen another injury like his throughout his career.
"To come in here, you want to have a nice few days to show everybody what you are capable of, to show why they have put faith in you and then the first thing that happens is you break your scapula," Merino said.
"It is tough to recover from that mentally. Everybody here in the club was really nice to me, really lovely, you could feel the love and support of everybody.
"It was such an unlucky moment. If you see the action... it's just... I mean... I haven't seen an injury like this ever in my career. But, well, these are things that happen."
Despite his unfortunate start to life in an Arsenal jersey, Merino adopted a positive mindset towards his injury, which he claims allowed him to adapt to life in North London and help him become a better player overall.
"Obviously it was not the best timing, but at the same time I like to look at the pros, the bright side of it. This gave me a lot of time to think," the midfielder added.
"To get to know my team-mates, to get to know the staff, to work on myself, the things that I need to do in terms of tactics, in terms of physicality and I think I am a much better player now than I was four weeks ago."
How are Arsenal's other injury victims progressing?
Merino's return comes at a time where Arsenal are already coping without captain Martin Odegaard, who is expected to be sidelined until after the next international break due to an ankle sprain.
Oleksandr Zinchenko (calf), Kieran Tierney (hamstring) and Ben White (groin) are also in the infirmary, and it remains to be seen if the latter's issue will be serious enough to keep him out of the visit of Southampton this weekend.
However, Takehiro Tomiyasu is closing in on a return from a pre-season knee problem, having returned to training around the same time as Merino, while Jurrien Timber should be fine for the weekend despite being taken off as a precaution in midweek.