The former Hertha Berlin, Borussia Dortmund, Schalke, Tottenham Hotspur and AC Milan midfielder tore up his contract with the Spanish side last summer to facilitate a free move to Frankfurt. Not a cent switched hands, but Frankfurt gained considerably.
Boateng's last Bundesliga station was Gelsenkirchen, where he scored six goals in 46 appearances in two years sandwiched between spells with Serie A giants Milan. It is fair to say his past three years have been blighted by off-field incidents, though, making Frankfurt's move appear like a gamble. It was, however, a calculated risk which has paid off, richly.
Watch: Will Boateng make it into YOUR Team of the Season?
The 31-year-old has missed just three of the Eagles' Bundesliga fixtures this season – one for suspension and two due to a thigh injury. When available, his has been one of the first names Niko Kovac has put on his teamsheet.
With six goals – three of which secured a victory, and another earned a draw at Hoffenheim on Matchday 12 – Boateng's contribution to Frankfurt pushing for a place in Europe next season has been substantial. He also appeared in every step of the Eagles' flight to a second straight DFB Cup final since signing up, even captaining the team in their second-round win over Schweinfurt.
Yet there were considerable doubts when he arrived.
"If you don't mature when you grow older, then you've got a problem," Boateng said, being frank about the image he had accrued. "When I read Bad Boy today, then that's just somebody opening an old drawer because they have no other ideas: people who know me now should be writing differently."
The 2017/18 campaign has seen Boateng come of age. He has been Eintracht's engine, pendulum and pivot, always at the heart of the action, receiving and spreading, setting up and scoring. He has showcased all the talent he had previously produced "in flashes", consistently.
"You've got to see Kevin as a footballer who has dozens and dozens of ways of getting past his opponent," said current Liverpool manager Jürgen Klopp during their Dortmund days. "Kevin can control the ball in a hundred different ways, yet he wasn't able to read the game well tactically - although he was working like crazy on learning this. His talent can be seen in flashes, but to become an all-round great footballer, you need to develop."
Watch: How Kovac and Boateng got the Eagles flying
It was a prophecy which Boateng would later acknowledge he had read and reacted to, working hard on improving all areas of his game, and the results were there for all – in particular Frankfurt fans – to see and enjoy this season. His understanding of the game enabled him to dictate the pace of the Eagles' play, as Kovac's extended arm on the field. No wonder the future Bayern Munich coach always picked him.
The younger players flourish under his on-field leadership, avail of his charisma and profit enormously from his work ethic. He has become a beacon, on and off the field.
The headlines Boateng is now making off the field differ greatly from those in the past. He has earned widespread respect for his engagement in fighting racism, with both FIFA and the United Nations appointing him as ambassador for anti-racism, while he addressed a Human Rights panel at UN headquarters in Geneva.
Outspoken and exuberant, Boateng now knows the time and the place for both of those key characteristics to be beneficial to the cause.
While he recently suggested that "90 percent of our success is down to Niko Kovac," the modesty of that remark masks the truth. That Frankfurt fans have another European tour to look forward to in 2018/19 is as much down to Boateng's brilliance as it is Kovac's cunning. Kovac has called the shots, but without Boateng delivering and distributing them, such success would not have been possible.
Frankfurt's most travelled midfielder will be the flagbearer again next term, within and beyond Germany's borders, and the Frankfurt faithful know who they should thank when they look forward to August's European draws and plan their autumn adventures – a free transfer paying dividends.