Italian Ancelotti did not divulge the nature of the injury although local media reported the Portugal captain had felt discomfort in a tendon in his left knee since the Champions League win at home to Schalke 04 on March 18.
Ronaldo is having treatment and training normally and will be available for Real's La Liga game at home to Rayo Vallecano on Saturday.
He is also the top scorer in this season's Champions League with 13 goals and Real host Borussia Dortmund in a quarter-final first leg on Wednesday.
"He has a small problem but it is not having any negative effect at all," Ancelotti told a news conference.
"I don't think it affects him on the pitch so he'll play tomorrow as normal."
Asked whether Real had considered resting Ronaldo, who has scored 27 La Liga goals this season, Ancelotti said: "It is something we have considered but he doesn't need it at the moment as it's a problem that is being dealt with very well.
"He is confident and is not complaining about it. But obviously if things get worse we will rest him but at the moment the player is calm and everyone is calm because it really is a small, small problem.
"He wants to clear it up in the coming days by playing and having treatment."
Real's title chances took a severe blow over the past week when they were beaten 4-3 at home by champions Barcelona and lost 2-1 at Sevilla.
Atletico Madrid are top on 73 points ahead of their game at Athletic Bilbao on Saturday with Barca, who play at city rivals Espanyol, a point behind in second. Real have 70 points in third, 14 ahead of fourth-placed Bilbao.
Ancelotti said Croatia midfielder Luka Modric had a fever on Thursday night and a decision on whether he could play against Rayo would be taken on Saturday.