Liverpool goalkeeper Loris Karius was referred to medical experts in Boston after contacting the club while on holiday in the United States.
The 24-year-old German visited Massachusetts General Hospital and was diagnosed with concussion five days after the Champions League final.
Karius made errors for the first and third goals his side conceded in the 3-1 defeat by Real Madrid on 26 May.
Dr Ross Zafonte said it was “possible” the injury “would affect performance”.
Shortly before his mistake for Real’s opening goal, Karius collided with Spanish defender Sergio Ramos – but in a statement released with the player’s permission, doctors did not say if that incident was the cause of the concussion.
Karius appeared to show no signs of injury and did not request any treatment during the game in Kiev, Ukraine.
The collision occurred early in the second half. Minutes later, the German threw the ball into the path of Real striker Karim Benzema, who stuck out his leg and scored.
Real’s third goal came when Karius let a long-range shot by forward Gareth Bale slip through his hands.
Dr Zafonte, who is a leading expert in head injuries in the NFL, said Karius’ assessment involved reviewing “game film”, a “physical examination” and “objective metrics”.
He added it was “likely” that “visual spatial dysfunction” – which hampers a person’s ability to process visual information about where objects are in space – would have occurred immediately after the event that caused the concussion.
Dr Willie Stewart, a consultant neuropathologist at Glasgow’s Queen Elizabeth University Hospital, said a delay in the concussion diagnosis is “not surprising”.
He told BBC Sport: “How did he get concussed and we didn’t know?
“The obvious signs of concussion can take hours and days to develop, so it’s not surprising that it might be picked up after the match.”
‘Football needs urgent review’
Following the incident, brain injury association Headway has called for football’s concussion protocols to be “urgently reviewed”.
New head injury measures were introduced by the Premier League and European governing body Uefa in 2014 which said only the doctor can decide whether a player should continue after a potential concussion.
Premier League rules were updated after Tottenham received criticism for allowing goalkeeper Hugo Lloris to continue playing after losing consciousness.
Headway chief executive Peter McCabe said: “It is alarming to hear that a player has played nearly half a match with a potential concussion.
“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.”
In the aftermath of the game, Karius received death threats, prompting a police investigation, and the goalkeeper told fans he was “infinitely sorry”.