Merino is going to really be a good addition to Arsenal,and with the sale of Smith Rowe already from the club,Merino will be a very quality signing judging from his performance in the Euro.He plays a similar pattern of football like that of Granit Xhaka who left the club last season and the club had struggled to get a replacement for him.
Merinos signing will really boster Arsenal's midfield,and couple with the fact that he will operete from the left flang,it will make Martinelli or Trosard get more goals next season.
I also think Merino will be great addition to Arsenal team. He is joining at right time especially since Smith Rowe left. Merino played really well in Euro showing how good he is. He plays in similar way to Granit Xhaka who left the team so he will fill a big gap. Merino will make Arsenal midfield stronger and he will help Martinelli and Trossard score more goals by playing on the left side like you said. I think Merino could make big difference for Arsenal and I'm excited to see how he will do next season.