The Spanish club confirmed on Thursday that the 23-year-old centre-back has injured the rectus femoris muscle in his right leg.
"He has been diagnosed with a muscle injury in his right leg. Awaiting progress," Real Madrid said in a brief statement.
While the club did not provide a timetable for his return, reports in Spain suggest Asencio could be sidelined for several weeks, making him a doubt for the start of the new campaign.
The injury comes after a difficult season for the young defender, who made 23 La Liga appearances despite battling several fitness problems. He missed matches after suffering a head injury in February before later being troubled by calf issues.
Asencio had been hoping for a strong pre-season to establish himself in Real Madrid's defence, but the latest setback is expected to delay those plans.
The defender has also been linked with a move away from the Spanish giants, with several Premier League clubs reportedly monitoring his situation.
Real Madrid play their next match against ACF Fiorentina in a pre-season club friendly on Saturday at the Worthersee Stadion in Klagenfurt, Austria.
Los Blancos' first La Liga match of the 2026-27 season takes place on Saturday, 22 August against RCD Espanyol at the RCDE Stadium in Barcelona.
Trent Alexander-Arnold produced his best performance of Real Madrid's pre-season campaign as he inspired a convincing 4-1 win over Leganes, showing he is ready to fight for his place following the arrival of Denzel Dumfries.
Karim Adeyemi has said joining Barcelona was an easy decision after receiving a call from head coach Hansi Flick, insisting he never needed convincing to complete his dream move to the Spanish champions.
Fulham are advancing in negotiations with Real Madrid over the permanent signings of striker Gonzalo Garcia and attacking midfielder Cesar Palacios.
Manchester City midfielder Rodri is facing a race against time to be fit for the start of the new Premier League season after undergoing minor back surgery.
