Casemiro was reportedly hurt by Erik ten Hag's decision to leave him out of the FA Cup final.
The Brazilian was initially named on the bench for Saturday's 2-1 win over Manchester City but was replaced by 19-year-old Willy Kambwala.
Erik ten Hag said that the midfielder was not 100 per cent fit for the match, although ESPN Brazil disputed this.
GOAL reports that Casemiro was hurt by the decision to exclude him from the squad for the final, according to their sources close to him.
Casemiro had started 10 consecutive games despite a difficult run of form, filling in at centre-back, but Lisandro Martinez and Raphael Varane returned as a pairing for the final.
Casemiro was reportedly hurt when Erik ten Hag dropped him for the FA Cup final on Saturday
Ten Hag said that Casemiro was injured and selected Willy Kambwala in his place on the bench
There are doubts over Casemiro's future at Old Trafford after a drop-off in form this season.
Three Saudi Arabian sides - Al-Nassr, Al-Ahli, and Al-Qasidiya - are all interested in him, GOAL reports, with the latter two the most keen.
They will also reportedly be willing to offer him a better package than the £350,000-per-week that he earns at United.
He appeared to miss the celebrations that his team-mates enjoyed after beating City, instead being pictured on social media jetting off on holiday with his family.
Casemiro's wife Anna Mariana shared a photo of the couple on a flight with their two children, Sara and Caio.
Anna Mariana, who married Casemiro back in 2014, captioned the photo 'gone on vacation'.
Missing out on the FA Cup final represented a disappointing end to the season for him.
The 32-year-old has struggled this season following an excellent first campaign following his move to United from Real Madrid.
Three Saudi Arabian clubs are reportedly interested in bringing in Casemiro this summer
Man United's players partied long into the night after they beat Man City to win the FA Cup
Casemiro appeared to be absent from the celebrations as he jetted off on holiday
Amad Diallo's live stream captured some of the dressing room celebrations at Wembley
Despite featuring in 32 matches - scoring five goals and contributing three assists - Casemiro has faced criticism for his sluggish displays.
He fired back at his critics in an interview with Sky Sports earlier this month.
Casemiro said: 'I was considered one of the best signings of the Premier League last season, and now I'm not worth anything anymore?
'The criticism is disrespectful. So when it's lacking respect, then it's worrying, and I don't have to respect that either'.
He slotted in at centre-half in a number of games towards the end of the season amid a defensive injury crisis for Erik ten Hag.