The Madrid talisman reportedly left training after just 20 minutes and it is now unsure if he will be fit to face the Bundesliga giants
The Portuguese star left the training ground after just 20 minutes on Monday with pain in the patellar tendon in his left knee, the same injury he suffered in the 3-0 win in the first-leg against the Bundesliga giants and also kept him out of his side's fixture against Real Sociedad at the weekend.
Ronaldo was treated by the club's physiotherapist immediately after leaving the field and, while the club have not officially ruled him out of Tuesday's match, Carlo Ancelotti did insist earlier that he would not feature unless he was "100 per cent" fit.
"Regarding Cristiano, he feels that he is at 100 per cent, so we are confident that he will feature," the former AC Milan boss told reporters beforehand. "We will not risk him, though.
"Cristiano always wants to play and I want him to play. But he is a professional and knows when he can't."
With a three-goal lead in hand, Madrid appear to have one foot in the semi-finals.
However, Ancelotti is taking nothing for granted, primarily because Dortmund will be buoyed by the return of star striker Robert Lewandowski from suspension.
"We are close to reaching the semis," Ancelotti acknowledged. "It's the biggest motivation you can have.
"But Lewandowski returning makes this a different game. His presence lifts his fellow players."
Lewandowski scored all of Dortmund's goals in their 4-1 first-leg victory over Madrid in last year's semi-final tie between the two sides.