Ronaldo has missed the past fortnight due to a hamstring strain.
Allegri said, “Unless something happens, Ronaldo will play. It's a pity not to have Chiellini and Emre Can, but the others are fine. Having Douglas Costa back is important, Paulo Dybala had a good training session this morning. Rugani will play at centre-back, it's an important opportunity for him and we're happy.
“It's true Atletico are a completely different team to Ajax, who are more open, so we've got to be smart in defence."