Casemiro was initially named on the bench for the clash against Manchester City, but it was revealed shortly before kickoff that his spot had been taken by Willy Kambwala.
Man United head coach Erik ten Hag claimed that the Brazil international was not 100% fit for the encounter, which led to Amrabat featuring in central midfield alongside Bruno Fernandes and Kobbie Mainoo.
However, according to former Watford player Nordin Amrabat, who is the brother of the on-loan Man United midfielder, Casemiro decided to sit in the stands after being dropped from the XI.
"Casemiro was not injured in the FA Cup final. He found out that he wouldn't start, so he thought to himself, okay I will sit in the stands then. He was fit to play," Nordin told Z Select.
Casemiro's future is the subject of much speculation
The 32-year-old made 32 appearances for Man United during the 2023-24 campaign, scoring five goals and registering three assists in the process.
Casemiro was used as a centre-back towards the end of the season due to the team's injury problems in that area of the field, but Lisandro Martinez and Raphael Varane were given the nod in the middle of the defence against Man City after returning from issues, and the pair helped their side record a 2-1 win.
The Brazilian has another two years left to run on his contract at Old Trafford, with the option of an additional 12 months, but he is being heavily linked with a summer exit.
Casemiro is the highest earner at Man United, collecting in the region of £350,000 a week, and it is understood that the club are determined to remove his salary from their budget this summer.
Al-Nassr, Al-Hilal 'lead the race for Casemiro'
The midfielder's future could be in the Saudi Pro League, as both Al-Nassr and Al-Hilal are said to be determined to secure his signature during the summer market.
The two clubs are allegedly prepared to offer Man United a healthy transfer fee, while he could also increase his already-substantial wage packet by moving to the Saudi Pro League.
Donny van de Beek and Christian Eriksen are two more midfielders expected to be on their way out of the 20-time English champions during the summer window.
Meanwhile, Amrabat's loan switch from Fiorentina will expire at the end of June, and it is understood that he will not be moving to Old Trafford on a permanent basis.
Man United finished eighth in last season's Premier League table but qualified for the group stage of the 2024-25 Europa League courtesy of their triumph in the FA Cup final.