The Spanish club head to the Westfalenstadion needing simply to protect a three-goal advantage generated by a 3-0 first-leg victory to book their place in the semi-finals.
But Portugal international Ronaldo, who has scored 14 goals in the competition so far this season, could miss another opportunity to extend his tally and rubber-stamp Real's progress.
Pictures published on marca.com on Monday showed Ronaldo hobbling off the pitch in Germany and boss Ancelotti is not willing to take any risks with his key asset ahead of the Primera Division run-in.
The forward was rested for Saturday's 4-0 victory at Real Sociedad due to concerns over his knee.
Ancelotti told realmadrid.com: "He is being treated and yesterday he trained hard. We'll see what happens in training today and decide whether to take any risks.
"If he's 100 per cent he'll play, otherwise he won't. All the matches he's played in, he has done so because he was fit. We have never taken risks with him or any other player.
"For me, he always has to play and therefore he and I have the same idea, but if he's not fit he can't play. He is very professional and knows his body well. When he does not feel fit he says so.
"That's what happened in the match against Real Sociedad."
Marcelo is likely to miss out again for Madrid due to a hamstring injury while Dortmund are still without the long-term injury absentees Jakub Blaszczykowski, Neven Subotic, Ilkay Gundogan, Sven Bender and Marcel Schmelzer.
The Bundesliga club's Polish striker Robert Lewandowski insists his side can pull off the seemingly impossible and overturn the three-goal deficit on home turf.
Lewandowski, who missed the match at the Bernabeu through suspension, is looking for a repeat of his heroics from last season's semi-final when he scored four times in a 4-1 first-leg win over the same opponents.
Not even a repeat of that result would send the Germans into the last four this time, though.
Only Deportivo La Coruna have managed to come back from such a deficit in the Champions League, recovering from a 4-1 defeat at AC Milan to win the home leg 4-0 in 2004.
"We need to write history again," Lewandowski said.
"If we can get an early goal, then there's a small chance. We mustn't be afraid.
"We have a minimal chance. It would be crazy to think that I could score four goals again, but we are always dangerous in Dortmund."
Dortmund defender Mats Hummels summed up his side's chances by telling Germany's Kicker magazine they were somewhere in the region of "three per cent".
"The most important thing is we try to win and show our fans the passion we have for the game," said coach Jurgen Klopp.
"We've developed a knack of doing well in the second half lately - well this is the second half of the tie."