The 24-year-old goalkeeper visited Massachusetts General Hospital in the United States and was diagnosed with concussion five days after the Champions League final.
The hospital's head injury specialist, Dr Ross Zafonte, said in a statement that the concussion "likely occurred immediately" following the second-half collision with Ramos.
"It is alarming to hear that a player has played nearly half a match with a potential concussion," Headway chief executive Peter McCabe told BBC Sport. "It is also very worrying that it has taken so long for this to be identified.
"Why wasn't the game stopped immediately so he could be examined on the pitch and what role did the fourth official play? Why wasn't he assessed immediately after the game or on his return to Liverpool?
"Football has recently made good progress regarding concussion but if mistakes have been made, especially in such a high-profile game with the world looking on, it is only right that the concussion protocols should be urgently reviewed and this matter given the correct scrutiny."