The Ghanaian, 27, has just come off the back of his best season in LaLiga and Atletico had been working on a new deal to tie him down until 2026 when the Gunners eventually stumped up the funds at the eleventh hour.
Atleti had planned to increase his clause to a more prohibitive €100m (£87m) as part of the fresh terms.
So what is all the fuss about? Thomas, as he is known in Spain, has taken time to reach his full potential but became indispensable at the heart of Atletico’s midfield under Diego Simeone over the past couple of years.
After signing terms with Atletico as an 18-year-old in 2011, the Ghanaian went out on loan to Mallorca and Almeria before making his debut with the Rojiblancos in 2015.
Initially on the fringes, he grew in importance for Atleti in recent seasons and especially since the sale of Rodri to Manchester City last summer.
A defensive midfielder in the mould of Arsenal legend Patrick Vieira, Thomas is not only strong and difficult to shake off in possession, but excellent with the ball at his feet and adept at carrying it forward.
Positionally, he improved immensely under Simeone and has a knack of being in the right place at the right time to win the ball back with key interceptions and move the team up the pitch with his dribbling ability.
“He was fantastic,” Simeone said after his man-of-the-match display in the 0-0 draw against Real Madrid last September. “Bringing the ball out from the back and initiating the play.
“He’s growing and I’m very happy for him. He was one of the players who gave us the most continuity in our play.”
It was not the first time the Argentine had lavished praise on the 26-year-old.
“Thomas is extraordinary,” he said last year. “If he came from [Manchester] City or United, we would all be talking of a fantastic player. But as he came from the youth system, it’s not said as much. So we say it: he’s fantastic.”
That was after Thomas had made a trademark 40-yard run to win the ball back for Simeone’s side in an impressive performance against Getafe.
But the Ghanaian, despite earning a reputation as a midfield shield, has the ability – and the energy – to pop up all over the pitch and is the type of box-to-box player who can now thrive in English football.
Although not prolific, he chips in with several goals per season. Playing for Ghana, he has also shown he can assume added responsibility in attack and has 10 goals in his 27 caps.
A number of his strikes have also been blockbuster efforts – powerful shots from outside the box with his right foot. And in a more attacking team than Atleti, he would be an even greater threat at the other end of the pitch.
At €50m, he represents a bargain and has his best years ahead of him, which is why Atletico opened talks to extend his contract before Arsenal swooped on deadline day.
He said last summer: “The Premier League is a good league, it’s very competitive and has some of the best players in the world. Hopefully, one day. I would like to play there.”
He has got his wish, and is unlikely to disappoint.