The Portugal forward was left limping in the final minutes of Wednesday's UEFA Champions League Group B clash in Sofia - which Real won 2-1 thanks to a late strike from Karim Benzema - with an apparent ankle injury.
Ronaldo had earlier cancelled out Marcelinho's sixth-minute opener with the second of two spot-kicks, the first of which was saved by Ludogorets goalkeeper Vladislav Stoyanov.
Former Manchester United star Ronaldo now has 14 goals in all competitions this season, and Ancelotti was quick to reassure fans that the blow is unlikely to stop him from continuing his fine form.
"Cristiano has had a knock," Ancelotti said. "It's painful but nothing serious."
Despite the unconvincing nature of Real's victory against the tournament debutants, Ancelotti was pleased with his side's showing, although he credited Ludogorets for their proficiency on the counter.
"We showed good intensity and created many chances," he added. "We should have scored earlier.
"We played 4-4-2 with [Gareth] Bale and Isco on the wings and we did well. Ludogorets were fast and counter-attacked well."
