Dutch defender Ron Vlaar is rated a doubt for Wednesday's World Cup semi-final against Argentina with a knee problem, Dutch officials said Sunday.

Vlaar has impressed in an Oranje side which bulldozed its way through the group stage, starting with a 5-1 thrashing of Spain, then seeing off Mexico and Costa Rica. The 29-year-old former Feyenoord star is out of contract at the end of next season and media reports say Manchester United are just one potential suitor.