The Uruguayan midfielder joined Atletico on a season-long loan on deadline day, which paved the way for Arsenal to complete their £45 million deal to sign Thomas Partey from the Spanish club.
Torreira joined Arsenal from Sampdoria in 2018 and helped the club win the FA Cup under Mikel Arteta last season.
The 24-year-old also came close to winning the Europa League during Unai Emery’s reign and endeared himself to Arsenal fans after he broke down in tears during the 4-1 defeat to Chelsea in the final.
And following his departure this week, Torreira has hailed Arsenal and says the club is ‘forever in my heart’.
‘It was an absolute privilege playing for Arsenal these last two seasons! Thanks a lot! We catch up soon!’ Torreira posted on Twitter.
‘Once a Gunner…always a Gunner!!’
Arsenal reportedly angered Atletico during their negotiations for Torreira’s loan but the Gunners have managed to secure a key advantage in the deal as the Spanish club do not have a purchase option next summer.
‘Lucas’s operation was very complicated,’ Torreira’s agent, Pablo Bentancur, said earlier this week.
‘Arsenal were targeting Thomas Partey and when the two clubs have a different objective it is difficult to be in the middle of the two operations.
‘We had other agreed-upon teams such as Roma, Fiorentina and Lazio, who play the Champions League, but Lucas was blunt. He told us that his goal was Atletico and that we get it for him and we work for the players.
‘He closed all the other possibilities, it either, ‘yes or yes’ to Atletico. Luckily in the end it was done.
‘They have agreed a loan for a year without a purchase option and the price is not defined, it will depend on his performance.
‘There is no option to buy because Arsenal had to pay the clause for Thomas.’