The 36-year-old left Shandong Taishan in November after four years with the Chinese club and has now decided to bring his 18-year career to an end.
Mourinho managed Fellaini for just over two seasons during their time together at United and the Portuguese, who was sacked by Roma last month, dedicated a special message to his former player.
‘I don’t forget who gave me everything he could give. You were and will always be one of mine,’ Mourinho wrote on Instagram.
‘Congratulations on your career and rest those ankles that suffered a lot.’
During his time at United, Fellaini helped the club win the FA Cup, Carabao Cup and the Europa League.
‘I am writing this post to announce that after a career of 18 years, I am retiring from professional football,’ Fellaini said on Instagram.
‘What an incredible journey it has been! I am so grateful to have played the sport I love at the highest level.
‘I am proud to have represented Standard in Belgium, Everton and Manchester United in the UK, and Shandong Taishan in China.
‘It was an honour to have played for the Belgian national team at the Olympics of 2008, Euro 2016 and the World Cups of 2014 and 2018.
‘I have played in front of the best fans in the world, and I want to take this moment to thank all of them for their unwavering support throughout my career.
‘Your encouragement and passion have been a constant source of inspiration for me.
‘As I embark on this new chapter in my life, I look forward to new challenges, but be assured, I will continue to support the beautiful game in any way I can.’
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