Portuguese medical staff fear Real Madrid ace Cristiano Ronaldo could do lasting damage if he plays against Champions League opponents Manchester City.
Ronaldo, who missed victory at Real Sociedad, is battling to recover from a hamstring injury.
The team doctor of Portugal, Dr. Nuno Campos, said: "If the diagnosis is really a 'microrrotura', recovery lasts between 18 to 21 days, that is 12 to 14 to fully heal and another 5 to 7 days for muscle recovery work.
"That is to say, with these times, he should be out until May 10, the game against City would be lost and also the League game against Valencia on 8 May.
"It would be less if it were a contracture and he could play against City.
"If you do that, having a microrrotura, you risk aggravating the injury and jeopardising participation in the Euros."
Nuno Campos added, "Cristiano overcomes many limitations that are not normal, but this is a physiological issue. It is difficult to be able to play before 20 or 21 days. The treatment they say will do with stem cells may hasten recovery days, but few. It's complicated."