Sahin was a personal gamble by Mourinho, the former Borussia Dortmund midfielder being one of the flashpoints which led to Jorge Valdano's demise as general manager last year.
But arriving at Real carrying a serious knee injury left the Turk - and Mourinho - too much ground to make up. Sahin's Real career never got going and his coach now admits it's time to part ways.
Mourinho conceded last week: "We must be honest with the players.
"If there is a good opportunity that arises, he should use it and go to another club."
Fully fit since January, Sahin simply couldn't dislodge Mourinho's preferred choices in midfield. But he hasn't suddenly become a poor player overnight.
In Germany, he became a title winner and was named the Bundesliga's Player of the Year - all before turning 22. At 16, Sahin became the youngest player to ever make his Bundesliga debut and barely weeks later the league's youngest ever goal scorer.
Borussia Dortmund still think highly of him, making an enquiry in January and trying again earlier this summer.
"Everyone knows that the Spanish media exaggerate positive and negative," argues Sahin's former coach at BVB, Jurgen Klopp.
"Nuri is an impatient guy, but no one can and should forget that he was injured for a long time.
"For Nuri Sahin, I am always available. Yes, we keep in regular contact."
London has been mentioned as his next destination. Real have attempted to include Sahin in their bid for Luka Modric, while those close to him say he would jump at the chance to play with Tomas Rosicky at Arsenal.
"He is the best No10 BVB has ever had," smiles Sahin, recalling his days growing up in Dortmund.
"I was always a BVB fan, but there was a time when I didn't go to their games. When Rosicky was injured, I had no interest in going to the stadium. I just wanted to see him play."
Now injury free - and with a full preseason under his belt - Sahin would prefer to stay and prove Mourinho was right in bringing him to Madrid last year. He's employed a personal trainer and gave up his summer holidays to get himself fit for the start of preseason training.
"I am firmly convinced that next season will be mine. I want to win many titles with Real Madrid," he declared only last week.
Described as a "sensitive character", the doubts and criticism in the Madrid press has hurt Sahin and he desperately wants to repay Mourinho for the faith the coach has shown.
But should Luka Modric arrive from Tottenham, the competition for a place in the coach's midfield will only become more intense.
"What he has shown so far, in his few games, has not impressed me," claimed Valdano earlier this year. "I do think he is technically a very good player, but he does not have the skills to shine alongside (Xabi) Alonso..."
At Arsenal, Wenger has earmarked Sahin as the player to fill the void left by Jack Wilshere's injury problems. Despite the sniping from Valdano in the Madrid press, Sahin's passing range doesn't suffer in comparison with the game's best midfield schemers.
And if encouraged, his goal scoring ability will add something new to Arsenal's midfield. Sahin's shooting power has been a feared part of his game ever since he broke into BVB's first team as a 16 year-old.
Mourinho knows this and any loan arrangement won't be just to protect Arsenal from a recurrence of Sahin's knee problems. The master coach, knowing the commitment the midfielder has shown in returning to full fitness, still believes he can have a Real future - if not this season.