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The Brazil midfielder is set to join United in a £60 million deal after agreeing terms on a four-year contract, with the option for a further year.
Ancelotti was asked about Casemiro's future in a press conference on Friday and he revealed the 30-year-old has said goodbye to his Real teammates.
He then admitted he couldn't personally understand why Casemiro was keen to leave the La Liga and Champions League holders for a 'struggling side'. It's worth mentioning he never said United's name.
In quotes carried by the Mirror, Ancelotti explained: "Casemiro wants to try a new challenge, and I and the club understand that.
"After all, he's done for this club, we need to respect his decision. Casemiro is leaving but we have a lot of resources to replace him.
"There is no turning back, Casemiro has decided to try a new challenge. We want to wish him the very best. Just yesterday I realised that Casemiro is leaving, but our plans don't change - we'll compete for everything with or without Casemiro.
"I think Casemiro has been a key player for us, he was important to [Toni] Kroos and [Luka] Modric - the combination was key to success.
"Casemiro's teammates understand his decision, they respect it - it was his will to try a new challenge and we have to accept it. I can't reply to why Casemiro traded European Champions for a struggling side, it's his personal decision."
United were keen to strengthen in central midfield this summer and spent most of it chasing Barcelona star Frenkie de Jong.
Erik ten Hag worked with the 25-year-old at Ajax and considered him a vital part of how his United side would play.
That pursuit appears to have been dropped in favour of Casemiro however, who has 15 major club honours to his name.
United have added Tyrell Malacia, Christian Eriksen and Lisandro Martinez to their squad this summer and Casemiro will be their fourth signing.
The Red Devils are still interested in Ajax winger Antony too, but had a £67.6 million bid rejected on Wednesday.
Antony is thought to be keen on a move and didn't train with his Ajax teammates on Friday.
There is a chance he doesn't feature against Sparta Rotterdam on Sunday.