Ronaldo was substituted in the 80th minute of the 3-0 quarter-final first-leg victory at the Bernabeu after receiving treatment on the pitch for what looked like a problem with his left knee.
However, Ancelotti does not think it is anything serious, saying after the match: "He was bothered a bit by his knee, we needed to defend and to avoid any risks, we've substituted him. It's not a problem. He isn't injured."
Ancelotti did say, though, that they would not take any chances with the Portugal international, adding: "We'll see how it goes. At this moment we're not concerned by the niggle. If it continues, naturally we'll rest him but we're not thinking about that because the player is calm.
"There's not a problem at the moment."
Ronaldo has been in prolific goalscoring again for Madrid this season, and was on the mark against Dortmund to earn himself a place in the record books.
His neat finish to put Madrid 3-0 up saw him equal the record of 14 goals in a single European Cup season shared by Lionel Messi and Jose Altafini.