The Ghanaian expressed his gratitude towards Los Rojiblancos but Simeone's side remain upset over the manner in which he left the club.

Thomas Partey completed his move to Arsenal late on Monday night, which was Deadline Day in the 2020 summer transfer window. However, the Ghanaian midfielder remained silent about his Atletico Madrid exit until the following morning, breaking his silence to bid farewell to the Spanish club.

The player had long been on the Gunners' radar but appeared destined to remain at Los Rojiblancos after several failed attempts to negotiate a transfer. Simeone and Co. were adamant that the only way he could leave was if Arsenal paid his $58M release clause in full. That's exactly what the north London giants did on the final day, handing the money to La Liga to break the 27-year-old's contract.

The midfielder underwent his medical in Spain and was confirmed as an Arsenal player at exactly 6:15 PM ET. The following morning, Partey took to social media to bid his farewell to Atletico Madrid, penning a lengthy goodbye via Instagram.

"Dear Atletico fans, today I only have words of gratitude with you," the Ghanaian wrote. "For several years now, Atletico Madrid has been my home and a part of this family will always be with me. 

"I want to thank the club for the trust placed in me from day one, but above all, I want to thank you, this wonderful hobby that I accepted from the first moment and that has shown that it is both in good times and in bad times and you never truly stop believing. Atletico Madrid is not just a club, it is a family. Thank you."

While the 27-year-old expressed nothing but gratitude for Atletico upon his departure, the same cannot be said for the Spanish giants. According to AS, his exit is said to have left a bad taste in their mouth as they were left infuriated by the manner in which he and Arsenal went about the deal.

The two clubs built a cordial relationship over the discussions regarding Torreira's loan move to the Metropolitano Stadium. Despite this, neither the Gunners nor Partey informed Simeone's side about their intentions. In fact, the team were informed by La Liga that the release clause had been met only 32 minutes before the window shut close.

Release clauses are meant to be paid to La Liga directly, which means Arteta's side played by the rules. However, Atletico were expecting a courtesy call but received none. Their thoughts on the matter was reflected in their farewell post – if you can even call it that – regarding Partey, writing via their official website: "LaLiga informed Atletico de Madrid at 11.28pm on Monday that Arsenal representatives have paid Thomas Partey's release clause at the sports association's headquarters.

"The player has therefore unilaterally terminated his contract with our club, which ran until June 30, 2023."  

Regardless of Los Rojiblancos' feelings, Arsenal got their man. Partey is a quality addition and is one that will gravely bolster Mikel's midfield. The Ghanaian will wear the no.18 jersey and will join up with the squad for the very-first time after the international break.