I don't think Real Madrid will take the player into their club again. They don't have a good history of using old players and pay them high salary. Some legendary players had to leave Bernabeu in tears like Raul Gonzalez, Iker Casillas and career of Casemiro in Bernabeu already ended two seasons ago. If he wants to continue his career in Europe, I guess he has reasons like want to play at highest level as his preparation for World Cup 2026 with Brazil. If he can join that tournament, it will be his last World Cup for his national team.
The rumor reveals another fact about terrible transfer strategy of Manchester United. What did they think to spend very expensive cost to buy an old player like Casemiro? They must be realistic to know old players like this can lose best performances after one or two seasons.
Casemiro himself has enough reasons to stay at Manchester United. His desire to continue playing at the highest level and prepare for the 2026 World Cup with Brazil makes him needs to be a regular starting player. He gets this already at Manchester United which he has no guarantee that he will get it at another big clubs. This provides him a competitive environment and exposure for him to maintain his form and fitness ahead of the World Cup.