Real Madrid star Cristiano Ronaldo should miss at least three weeks due to his hamstring injury or risk aggravating it, a former Portugal team doctor said.
Ronaldo has been sidelined since late April but the La Liga giants remain hopeful he may be fit to face Manchester City on Wednesday.
However, former Portugal doctor Nuno Campos said if reports were true, the 31-year-old needed a break or risk a setback that would rule him out of Euro 2016.
"If the diagnosis is indeed a micro-tear, he needs between 18 and 21 days," he told newspaper Record.
"They need 12 to 14 days to heal completely and another five to seven days for muscle recovery work.
"It would be less time if it were just a contracture. If so, the problem does not stop him from playing against City."
Madrid's Champions League semi-final tie against City is delicately poised after a 0-0 first-leg draw at the Etihad Stadium.
But Campos said there was a concern Ronaldo could suffer further damage and miss the European Championship, which starts on June 10.
"If you do, having a micro-tear is subject to aggravate the situation and there is a risk to the European [Championship]," he said.