The experienced goalkeeper picked up a knock in Tuesday's Champions League encounter and could face a brief spell on the sidelines.
The 32-year-old featured from the start in Tuesday's Champions League encounter, but picked up a knock early on after an unfortunate collision with teammate Sergio Ramos.
Casillas initially appeared able to continue play after undergoing treatment on the pitch, but he was eventually replaced by Diego Lopez in the 15th minute.
"After the tests (CAT scan) at Sanitas La Moraleja Hospital the medical team have diagnosed a bruised left rib which will be treated," a statement on the Madrid website reads.
It's not yet known how long Casillas will be unavailable due to the injury, but his participation in this weekend's home Liga clash with Getafe is in serious doubt.
The club captain has lost his starting berth to Lopez, but head coach Carlo Ancelotti announced last week that the Spain international is likely to be first choice in the Champions League.