Arsenal made Thomas Partey’s Gunners shirt available to buy - before announcing his signing.
The official club shop website had Partey listed as No. 18 with the Gunners having paid his release clause to Atletico Madrid.
That was on the site before Arsenal actually confirmed the deal for the midfielder, 15 minutes after the transfer deadline of 11pm.
A move could be done with the Gunners having paid a release clause for a player they had been linked with for the majority of the summer.
Partey has been a long-term target for Arsenal but his €50m (£45m) release clause was deemed too hefty in recent months.
Mikel Arteta's side wanted to drive the Ghanaian's price tag down but were met with consistent resistance from Atletico.
Arsenal changed their stance, coughed up the required cash and landed the 27-year-old right before the summer transfer window slammed shut.
Arsenal Technical director Edu, said: “Thomas has all the right attributes to be a top player for Arsenal. He is a leader on and off the pitch and he is exactly the kind of player and person we want at the club.
“We already feel like we know Thomas very well based on the work we have done analysing his performances closely in recent times.
"With his all-round game and positive aggressive style, he will be a brilliant addition to our squad. We’ve made a strong start to the season and we want to continue to build on this with Thomas now part of our club.”
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While Gunners boss Arteta added: “We have been watching Thomas for a while, so we’re now delighted to add such a high quality player to our squad.
“He is a dynamic midfielder with great energy. He brings a lot of experience from a top club that has competed at the highest level in La Liga and the Champions League for several years.
"We’re very impressed with his attitude and his approach to the game. He’s an intelligent footballer and we’re looking forward to him integrating into our system and contributing to the progress we’re building at the moment at the club.”