The Premier League club signed the Ghana international defensive midfielder from La Liga outfit Atletico on summer transfer deadline day on Monday.
The Gunners had to pay the 27-year-old’s full release clause of 45 million pounds.
Arsenal Technical Director Edu - a former player for the North London outfit - has explained how the transfer came about.
Sky Sports quotes Edu as saying: “One day I would like to show the fans how we work, or how we worked, because if you saw my plan six or seven months ago when I did it with Mikel (Arteta), Thomas was there already. So you see how many months we had to try to find the right player for the club.
“So many things to do, so many meetings, we had to see how we were going to buy the player, if we had the financial situation to buy the player, the legal process, the financial process, talk to the player, talk to the agent, so there are a lot of things around this but in the end we did it!”
Arsenal, who will play in the Europa League this season, have made a strong start to the 2020-21 Premier League campaign.
Mikel Arteta’s side are fourth in the league table at the moment with nine points from four matches.