Since the departure of the duo of Ozil and febregas who did well as a creative midfielder, Arsenal had not fill that area in their squad, though Smith Rowe who is presently playing in that role had not been creating clear cutting passess for strikers that is usually synonymous with Ozil and Fabregas, he is more attacking and offensive style of play, though he is presently one of the best player in the squad, I think signing of Isco will be a great asset to the team, he is an experience creative midfielder possibly filled that role perfectly for Arsenal.
I believe that ISCO could be a great addition to Arsenal. He is only 29 years old and can play at least another 5 seasons competitively, we have seen Milan get so much out of Ibrahimovic even while he is 40 years old, so a 29 year old would be basically "young player" for Milans roster even if not for Arsenal but still decent for Arsenal as well. Combine Isco's great passing style with Smith Rowe's addition to offensive line then make it so much greater for them.
I loved Ozil and Fabre together but let's be honest today's game requires midfielders to be present at offense, passes are great and all but scoring is something expected from them so Smith Rowe is great for todays deal. Let's hope that they get him, and even if not then maybe someone like Isco so that Arsenal could get better overall. I doubt they would do it at the middle of the season though.