Real Madrid goalkeeper Iker Casillas was hit by an object thrown from the crowd during his side's 1-0 win at Athletic Bilbao on Saturday.
Objects rained down on the keeper's area during the match and television pictures showed what appeared to be a knife or bottle opener had also landed on the edge of the pitch.
'The atmosphere at San Mames is always very powerful and that zone is always like that,' Casillas told reporters. 'It is a strange way to support your team.'
The bus carrying Real Madrid players to the stadium was also hit by objects thrown by rival supporters, resulting in damage to its windows.
Meanwhile, Real Madrid striker Ruud van Nistelrooy has confirmed he is close to signing a contract extension with the Spanish champions.
'Father Christmas is going to bring me the contract extension,' the 31-year-old Dutchman was quoted as saying in sports daily Marca on Sunday. 'It's nearly done and I'm very happy about it.'
Van Nistelrooy's present contract ends in June 2009.