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Featured Image Credit: Alamy
Schweinsteiger arrived from Bayern Munich in 2015 and he's an undisputed legend of the game, having won the Champions League and the World Cup.
Mourinho took over as manager in 2016 and didn't treat Schweinsteiger with much respect, something he admits he regretted years later.
The former Germany international told the UTD Podcast: "It was very strange on my birthday I came to the dressing room wanted to change but then someone told me I'm not allowed to go into the dressing room and I had to change in the dressing room and had to play with the second team.
"I was very hurt because no one was talking to me. Then I was speaking to Mourinho about it and he explained something to me something which I really did not understand and it had to do with the second injury that I got.
"My first injury I got the season before I got treated by the medical department of United and then in an easy training session I got the same injury so I asked Louis Van Gaal if I could get treated in Germany because we had the Euros in 2016 coming up and it was always agreed to that.
"So I did that, I was fit, I played the Euros, I was looking forward to coming to United to play with Mourinho who twice actually wanted to buy me with Inter Milan in 2010 and then later on at [Real] Madrid."
Schweinsteiger barely featured under Mourinho and even the Old Trafford faithful were left puzzled by his treatment.
When he eventually made his first appearance of the 16/17 season, off the bench against West Ham United in the Carabao Cup, he was given an incredible reception.
The 37-year-old says Mourinho thought he was unprofessional for having his treatment abroad and didn't want anything to do with him.
He continued: "I had a lot of respect for him (Mourinho) obviously but I did not understand the situation why I was not allowed to train play with the first team because there were some different opinions about me being professional which is not true because I am professional - I showed it later on I was always training with the kids or alone."
Schweinsteiger won the FA Cup with United and joined MLS side Chicago Fire in 2017 before retiring in 2019.
Despite having left the Red Devils, Schweinsteiger also received a Europa League winner's medal.
Mourinho would be sacked in 2018 and replaced by Ole Gunnar Solskjaer.
He currently manages Serie A side Roma after a spell at Tottenham Hotspur.
Roma are sixth in Serie A and five points off fourth place Atalanta.