The Portuguese superstar suffered an apparent hamstring injury ahead of a return to action for Juventus.

Tuesday was not a great day for Cristiano Ronaldo. After his return to the national team ended in a disappointing 0-0 draw with Ukraine, his second match with Portugal -- a clash with Serbia -- did not go down any better.

The 34-year-old superstar was forced to leave the pitch early, after only 28 minutes. In an attempt to stop the ball from going out of play, the five-time Ballon d'Or winner appeared to injure his hamstring, grabbing the back of his leg as he dropped to the floor.

Ronaldo soon asked to be substituted, as Juventus fans watched on in worry. However, the former Real Madrid ace was quick to calm down fears after the game.

"I'm not worried," the five-time Champions League winner told reporters.

"I know my body and I know that I'll be fine within two weeks."

If true, Ronaldo's estimation means he should be back in time for the clash with Ajax. However, with only 15 days remaining before that first-leg trip to Amsterdam, his recovery will go down to the wire.

Ronaldo's mastery of the Champions League saved the Bianconeri from a round of 16 exit and earned them a spot in the last eight. Facing the Dutch giant without their top scorer will not be ideal. 

Allegri's men will face Empoli, Cagliari, and AC Milan in the Serie A before that much-anticipated European game.  Ronaldo will likely be rested for these games so he can be fit for the competition he reigns supreme in.