Cristiano Ronaldo is suffering from tendinosis in the region around his left kneecap, the Portuguese Football Federation (FPF) said in a statement on Wednesday.
The FPF said that Real Madrid forward Ronaldo had undergone "specially adapted, individual" training on Monday but did not give any further details.
The World Player of the Year missed last Saturday's 0-0 draw at home to Greece with what the FPF described as a left thigh injury.
Portugal are training near New York and face Mexico in a friendly in Boston on Friday.
Ronaldo struggled with minor injuries during the closing weeks of Real's season before playing 120 minutes in last month's Champions League final against Atletico Madrid and scoring the final goal in their 4-1 win with a penalty.
Portugal's opening World Cup match is against Germany in Salvador on June 16.