The Ghanaian midfielder has moved to North London as the Gunners agreed to pay his £45m release clause.
Libertad Digital reports the deal caught Atletico manager Simeone by surprise.
The Argentine was “so sure" that Partey would stay that he has been talking to Lucas Torreira – who joined from Arsenal on-loan in a separate deal – about a midfield pairing.
And it saw the boss mention the Ghanaian in tactical discussions with the outgoing Gunners star, starting the midfielder regularly since the start of the 2020/21 campaign.
With the 27-year-old now set for his £45m move to Arsenal, it has left the Atletico boss “very angry".