Currently, Real Madrid already has Tchouameni who can be played regularly if Casemiro really went to Man United. But there are rumors that Toni Kroos wants to end his career when his contract expires. So if the rumors come true then it's clear that Real Madrid will need a new midfielder. Well, maybe it would be best to prepare now or at least Real Madrid has signed a new midfielder before the season ends, which means it could be done in the upcoming winter transfer window.
Carlo Ancelotti have know about who will be replacement position leaving by Casemiro because he not try keep extended Casemiro few years later at Bernabeu. I think Real Madrid have many alternative for Casemiro position player from Tchouameni, Camavinga and move back Valverde from left midfielder become defessive midfielder. I think this reason why Ancelotti not worry after leaving by Casemiro. I think not really urgent for Real Madrid get new midfielder if above player keep consistent on top performance.