It seemed like a bizarre decision at the time - selling a striker like Luis Suarez to a title rival for a modest price of just £4million.
Last summer new Barcelona boss Ronald Koeman made it clear that the Uruguayan was not in his future plans and in swooped Atletico as they offered him a two-year deal to take him off their hands. A strange way for the club to end the stint of the third-highest scorer in their history.
We know Barcelona are in trouble financially and would have been grateful to get the 34-year-old's sizeable wage bill off their books, but it's a decision they must be looking back on with some regret now as the former Liverpool star fires Diego Simeone's men to the La Liga summit.
Suarez is top of the scoring charts in the Spanish top flight with 12 goals - one more than his old pal Lionel Messi, who was left furious by his sale and did not hold back in calling the decision to let him go 'crazy'.
'What happened with Luis Suarez to [Atletico Madrid] was crazy. He left for free, paying the remaining years of his contract and he joined a team that fight for the same objectives as us. Unbelievable.
'Luis Suarez's departure had nothing to do with the decision [to submit a transfer request] but I felt it was crazy the way his exit was handled and that Barcelona let him go to a direct rival. We speak a great deal and I speak to Luis every day.'
Crazy is not a bad word to describe it. Crazy to strengthen a direct rival while leaving yourself weakened. Suarez didn't have his most prolific campaign for Barcelona last season but he still scored 21 times.
Take that away and you've got a lot of burden on a Messi whose devotion to the cause is not what it once was, and an underperforming Antoine Griezmann when the Argentine occasionally has an off day.
Crazy given Barcelona ended up keeping Martin Braithwaite at the Nou Camp instead, the former Middlesbrough striker who signed on an emergency loan last season. Judging by his end product since signing on the dotted line, the need for his services looks like more of a false alarm.
In 27 La Liga appearances, Braithwaite has scored just three times - two of which have come in this season. To his credit, the Dane has notched six goals in all competitions in 2020-21, but most of his impact has come in cup games.
The 29-year-old has actually played more games than Suarez overall, statistics from Opta show, and he has been given 13 starts by Koeman - plenty of time to prove himself. Braithwaite on his day is clearly capable of getting on the scoresheet given the quality he has in support.
He scored a brace against Dynamo Kiev in November and followed that up with two goals in routs against Osasuna and Ferencvaros. But Barcelona no longer have the game changer they once had in Suarez.
Atletico always seem to have a star man up front to push them forwards - from Fernando Torres to Sergio Aguero or Radamel Falco and Diego Costa.
Now they have Suarez and the club have been left pinching themselves after witnessing his sublime form under Simeone. After Suarez notched his 12th of the season against Valencia, club president Enrique Cerezo called him 'the best centre-forward in Europe'.
He has five goals in his last five La Liga games with a minutes-per-goal rate of 118 and is proving to be deadly in the box too - he has a 34 per cent shot conversion having taken just 35 shots in the Spanish top flight.
Suarez has proven himself to be a divisive figure in his career - whether he's biting his opponents, diving in the box or deliberately handballing on the line, he is never far from the headlines.
You always know what you're going to get with Suarez, but last week Simeone explained that it was this fire in the belly that convinced him he would be a success at the Wanda Metropolitano.
'The moment I spoke with Luis on the phone I found what he gives on the pitch: a lot of desire and energy,' he said.
'The team helps him to be at his best, which is scoring, something that has marked him throughout his career.'
His success at Atletico will certainly leave Barca fans scratching their heads about their transfer policy. The Catalan outfit went on something of a cull last summer as they shifted the likes of Ivan Rakitic, Artur, Nelson Semedo, Rafinha and Arturo Vidal - all players who were on big salaries and not guaranteed minutes.
But it does seem baffling that Braithwaite survived the exodus while Suarez was given the chop despite playing 36 games in all competitions last season.
Some of the incomings have certainly been questionable - Miralem Pjanic cost around £60m but has started just three times, while £23.5m Serginho Dest hardly looks like a world-beater at right back.
But by far their worst piece of business was letting Suarez go - and it's having a real impact at the top of the table. There are ten points separating fourth-placed Barcelona and Atletico at the top.
Suarez has not just proven that his old club were wrong to get rid - he's showing that he still has plenty more years at the highest level. Barca are entering a new era and are not quite the team they once were - but they are significantly weaker without Suarez and their loss is Atletico's gain.