"I'm calm, because I know I'm going to be back well in one or two weeks with no problem".
Ronaldo, 34, pulled up clutching his hamstring after chasing a ball to the touchline and immediately signalled to the bench that he needed to come off.
"I know my body and in two weeks' time I'll be ready to play again", he said.
Though it remains to be seen how serious his injury is, in being forced off the pitch Ronaldo joins rival Lionel Messi on the sidelines, after the Barcelona wizard suffered a pelvic injury during Argentina's defeat to Venezuela.
The Bianconeri face Empoli when the Serie A season resumes on Saturday, with games against Cagliari and AC Milan taking place in early April before a trip to Amsterdam to face Ajax beckons on April 10.
But Portugal were controversially denied a penalty against Serbia despite the referee having pointed to the spot - with a chat with his assistant leading him to reverse the decision. In their last game against Ukraine, Portugal was the much better team with 59% of ball possession, 8 shots on goal, 18 corner kicks and 91% of correct passes but weren't able to score a goal and game ended in a 0-0 draw.
Serbia coach Mladen Krstajic played down the impact of Ronaldo's misfortune as he was quick to point out that his injury-hit side missed several key players.
Who will progress to the semi-finals out of Juve and Ajax?