According to Football Leaks (h/t Football Italia), the Spain international tested positive for Desametasone after the game, which Los Blancos won 4-1.
The allegations come as part of the latest wave of Football Leaks reports. Der Spiegel had previously announced it would reveal the name of a "multiple Champions League winner" who tested positive for doping.
Los Blancos have responded to the allegations (h/t Marca), denying Ramos broke the rules:
"In relation to the information published by Der Spiegel referring to our captain Sergio Ramos, the club states the following:
"1. Sergio Ramos has never breached the anti-doping control regulations.
"2. UEFA requested timely information and closed the matter immediately, as is usual in these cases, after verification by the experts themselves of the World Anti-Doping Agency, AMA, and of UEFA itself.
"3. On the rest of the content of the aforementioned publication, the club does not pronounce itself before the evidence because of the insubstantiality of it."
Per the report, Desametasone is a substance that can be used as long as UEFA are notified in time. That was not the case, but Los Blancos' chief medic took responsibility.
The Times' Martin Zygler shared part of the original Spiegel report:
A previous issue arose when the Real Madrid players only accepted a surprise doping test in February of 2017 if it was performed by club staff rather than the members of the anti-doping team, which is against regulations.
Per Der Spiegel, Ramos' compliance with doping authorities was again called into question when Real faced Malaga in April. The 32-year-old was asked for a urine sample and told he couldn't shower before providing it. Despite this, he showered in front of the officer.
The Spanish anti-doping agency took more than five months to handle the case, concluding no violation occurred.
Ramos is one of the most decorated and polarising figures in the sport. His aggressive playing style often leads to controversy, with perhaps the most notable example in the 2018 final, when he tangled with Liverpool's Mohamed Salah and injured the Egyptian in the process.
Sport Witness suggested the story would've gained more attention had the violation occurred after the 3-1 win over Liverpool:
In the 2017 final, Ramos was criticized after what appeared to be a blatant dive saw Juventus' Juan Cuadrado sent off. He also made headlines in early November after catching Viktoria Plzen's Milan Havel with an elbow, bloodying his nose.
While he frequently invites controversy, Ramos is widely regarded as one of the best defenders in the world. With four La Liga titles and four Champions League wins under his belt for Real, as well as a World Cup and two European Championships with Spain, few can match his collection of silverware.
Ramos has been named to FIFA's team of the year nine times.