Cristiano Ronaldo ended his participation in a Portugal training session early on Wednesday and left the pitch with ice on his knee, Portuguese media reported.
The website of the sports paper Record showed Ronaldo walking off with a large pack of ice attached to his left knee. There was no immediate comment from the Portuguese Football Federation (FPF).
Ronaldo struggled with minor injuries during the closing weeks of Real Madrid's season before playing 120 minutes in the Champions League final against Atletico Madrid and scoring the final goal in their 4-1 win with a penalty.
He was rested from Portugal's early pre-World Cup training sessions after being diagnosed with a thigh injury and tendonitis in his left knee.
He stayed on for the whole of Portugal's opening Group G game against Germany on Monday which they lost 4-0. (Reporting by Brian Homewood, editing by Ed Osmond)