His deal runs out at the end of June, meaning he'll leave on a free transfer. He has been among the club's top earners since his surprise switch from Los Blancos during Erik ten Hag's inaugural season as United manager. However, there's also a specific reason why United have made the 33-year-old's departure public now rather than waiting until the summer.
According to the Manchester Evening News, Casemiro requested the announcement be made immediately to prevent any misleading speculation about his future that could hinder discussions with prospective clubs. It's believed he is currently earning more than £300,000 weekly at United.
Casemiro's departure means the next United manager will have an additional £15.6m annually available for their wage budget. Meanwhile, it's also understood Casemiro was to receive another £2.6m had the Reds secured Champions League qualification.
Announcing this now ensures the club won't be obliged to pay Casemiro this bonus. It may also provide relief to interim boss Michael Carrick, as it means his Champions League qualification ambitions won't carry any financial implications for the club, should he manage to deliver it.
His long-term prospects at Old Trafford have been uncertain since Sir Jim Ratcliffe arrived with plans to recruit younger talent and reduce the wage bill. Nevertheless, it was a showing that demonstrated Casemiro remains a valuable asset.
Notably, his contract contained a provision whereby a one-year extension would be activated if he made 35 starts this season. He would need to feature in every fixture from this point until the campaign's conclusion for this clause to come into effect.
If there was any remote chance of this occurring, it was effectively ruled out by the club's announcement. In his own statement to United supporters, many of whom have held differing views on the South American's impact, he said: "I will carry Manchester United with me throughout my entire life.
"From the first day that I walked out at this beautiful stadium, I felt the passion of Old Trafford and the love that I now share with our supporters for this special club. It is not time to say goodbye;.
"There are many more memories to create during the next four months. We still have a lot to fight for together; my complete focus will, as always, remain on giving my everything to help our club to succeed."