Marca says he has an EU passport, although he is clear that his national team, despite his Polish origin, is the United States.
Real Madrid's interest has been reported by Fabrizio Romano and Slonina fits the profile of the modern goalkeeper, that is to say, close to two metres tall (1.93 centimetres), powerful and possessing good ball skills.
In the event that the deal is completed, his destination would be Castilla, but it is not ruled out that he could be one of the first team goalkeepers in the almost certain event of Andriy Lunin departing in search of playing time.
A number of Premier League clubs have been looking at Slonina during the season, with Chelsea being the closest to signing him. However, the London club's sanctions put the brakes on the move, which has allowed Real Madrid and two other unnamed Premier League clubs to enter the equation.