Illarramendi was bought for almost 40 million euros ($45 million) when taxes were applied to his transfer in the summer of 2013. That was a lot of money for a 23-year-old defensive midfielder who many believed was not even the best player in the Real Sociedad squad at the time.
Fresh off helping Spain's Under-21 side to the European Championship crown in Israel in 2013, Illarramendi joined Real but took time to settle in the capital because of homesickness and something of a culture shock after moving from a small town with 5,000 inhabitants (Mutriku) to Spain's largest city.
On the pitch, he struggled to make an impact as well. Limited in terms of technique and barely venturing forward at all, the Basque did not seem to offer much in the way of protection for his defense either and was woeful when Madrid needed him most — the night it lost 2-0 at Borussia Dortmund in the Champions League in 2013-14 and almost saw its Decima dream go up in smoke.
After Illarramendi's poor performance in that match, which saw him taken off at the break, coach Carlo Ancelotti appeared to lose faith. When Alonso left, not only did Illarramendi fail to emerge as Xabi's successor, but he featured even less than before.
In his second season, Illarramendi played 41 games but only 15 as a starter. As he returns to Real Sociedad at a discounted price of €16 million, there is a sense that he never really came out of his shell in Madrid.
At €39 million, the transfer cost Los Blancos €19.5 million per season, €433,333 per game (for 90 in total) and €9,213 per minute (4,233 played). It is far from the return Real had expected on its significant investment — and is also a blow to the club's plans of recruiting the best young Spanish players around.
The two-tier policy of president Florentino Perez is designed to bring in the finest footballers from around the world and complement those with Spanish stars. But for 14 minutes late in the game against Sporting Gijon on Sunday, there was only one Spaniard on the pitch for Madrid: captain Sergio Ramos.
Goalkeeper Iker Casillas left earlier this summer, while David de Gea's proposed move from Manchester United remains in limbo. Jese and Isco, meanwhile, had been substituted and Dani Carvajal came off the bench later.
Although not really up to standard, Illarramendi is the latest casualty of a chaotic transfer policy. Lucas Silva was brought in last season but now is seemingly surplus to requirements, as he is close to a move on loan to Marseille. Inter's Mateo Kovacic signed recently and is likely to be used in an unfamiliar defensive midfield role.
"I was told that I was in the club's plans," Illarramendi said Wednesday. "Then they signed another player."
Nevertheless, it was probably the right time to let the player leave. Never really good enough and certainly not worth anywhere near the enormous outlay, the 25-year-old departs on the same day as another overpriced player, left back Fabio Coentrao, who moves on loan to Monaco.
Coentrao has been a useful player these past few seasons, but never lived up his hefty price tag after he was brought in by Jose Mourinho in a €30 million deal from Benfica in 2011, making him the world's most expensive fullback.
With constant coaching changes in recent seasons and a president who wants a big-name forward to arrive each summer, Madrid's policy in the transfer market is still something of a scattergun approach. Whereas Barcelona strengthened key positions last season and went on to win the treble, Real still seems to be placing square pegs in round holes, with players like Isco, James Rodriguez, Gareth Bale, Jese, Toni Kroos and now Kovacic all playing out of position or operating in less-familiar roles.
While Illarramendi wasn't good enough in his two-year spell at the Bernabeu, his growth was also stunted by the club's stockpiling of midfielders in recent seasons, which means there is no place in the starting lineup at times for players like Rodriguez or Isco.
As he returned to Anoeta after two years away, Illarramendi was welcomed by fans and former teammates alike. "We do love you here," defender Inigo Martinez wrote on Twitter, seemingly implying that was not the case at Madrid.
Given time to find his feet and confidence with a starting spot, Illarra will now probably produce his best football again in the Basque Country. Madrid, meanwhile, will reflect on a €23 million loss on a player the club should never really have bought in the first place. But Los Blancos would also be advised to ponder their many mistakes made in the transfer market over the past few years.
Time for a rethink, perhaps.