Lisbon:Dutch striker Ruud van Nistelrooy may face a UEFA investigation after reports that he was involved in a heated argument with a referee after yesterday's Euro 2004 semi-final defeat by Portugal. Nistelrooy reportedly said that he believed the official had favoured the tournament hosts in their 2-1 win. The Manchester United striker was furious with Swedish referee Anders Frisk's performance and had to be led away after confronting the match official following the final whistle. Frisk mentioned the confrontation in his report and UEFA have opened disciplinary proceedings against van Nistelrooy. They are expected to announce his punishment on Friday.