Casemiro had originally been named on the Manchester United bench for today's FA Cup clash with Manchester City.
However, the Brazil international will miss the showdown entirely after reporting a problem prior to kick off at Wembley.
Casemiro has been forced to play as a centre-back for United in recent weeks amid the club's torrid injury list.
Yet the Samba star had been left out of Erik ten Hag's starting XI for today, with Lisandro Martinez and Raphael Varane deployed in the heart of the Dutchman's defence instead.
Casemiro won't be involved in Man Utd's FA Cup final clash with Man City today
PA
Andre Onana starts in goal, with Martinez and Varane joined by Aaron Wan-Bissaka and Diogo Dalot in a back four.
Sofyan Amrabat gets the nod in midfield, along with Kobbie Mainoo and Scott McTominay.
And Bruno Fernandes starts as a false nine, with Marcus Rashford and Alejandro Garnacho deployed on the flanks.
As United prepare to battle City at Wembley, talk of Ten Hag leaving the club has ramped up.
Kieran McKenna, Mauricio Pochettino, Thomas Tuchel and Thomas Frank have all been linked with a move to United amid the uncertainty over Ten Hag's future.
But Ten Hag was in no mood to speak about his position at Old Trafford when quizzed by the BBC.
"I don't talk about my future. It's not about me, it's about the team and winning the cup," he said.
United lost to City in the final of the FA Cup 12 months ago.
Ilkay Gundogan bagged a brace for Pep Guardiola's side to secure the prestigious prize, with Bruno Fernandes on target for the Red Devils.
But Ten Hag feels the result can be different this time around with a 'good' mood around his squad.
"Two years after each other we're in the final but this year we want to win," he said.
"The mood is very good, strong spirit, strong belief, a tough opponent and we have to give everything we have, sacrifice, suffer but be brave and let them suffer as well.
"We have to be on the front foot and be aggressive. It's about mentality.
"If we have a strong belief and are brave then you have the opportunity. We had the opportunity last year but didn't take it.
"Casemiro is not 100 per cent and in that space around Phil Foden we need a strong defence."
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Man Utd have gone with Sofyan Amrabat instead of Casemiro for today's FA Cup final clash with Man City
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United legend Wayne Rooney, meanwhile, has opened up on Ten Hag.
He feels his old club must clarify the Dutchman's position as quickly as possible, admitting it provides a distraction for the Red Devils ahead of today's match.
"Whatever happens with Erik Ten Hag after the game, the club need to be quick on whatever happens, whether he stays or goes because I don't think it's right to let it linger on," he said.
"For the players, it's just to focus on the game today. It's a tough enough job as it is to beat this Manchester City team."
Rooney won the lot during his time at United.
Erik ten Hag seems to be in good spirits ahead of Man Utd's FA Cup final clash with Man City
GETTY
But he admits City are the team to beat these days, especially given how the Citizens recently won their fourth Premier League title in a row.
"It [the Manchester Derby] gradually started becoming where Man City were challenging us for league titles and there was more on the game," he said.
"It's a big game obviously and City have overtaken Man United in that sense and at the moment they're the biggest club in the city."