The Portugal international has been in scintillating form in 2013-14 and notched his 50th goal of the season in all competitions with a spectacular back-heeled volley against Valencia on Sunday.
But he suffered a suspected muscle problem in the opening stages of Wednesday's Valladolid game and was withdrawn after just eight minutes. Ronaldo was spotted pointing towards his thigh when he was replaced by Alvaro Morata.
The potential loss of talisman Ronaldo could not have come at a worse time for Real as they remain in the hunt for a La Liga and Champions League double.
Real have two league games left after their trip to Valladolid as Carlo Ancelotti's men head to Celta Vigo and host Espanyol before facing La Liga leaders Atletico Madrid in the Champions League final on May 24 in Lisbon.
With the FIFA World Cup in Brazil also little over 30 days away, the 2013 Ballon d'Or winner will be hoping for some positive news when he undergoes further medical checks.
