Ronaldo, a five-time European Cup winner, left the field with a thigh problem captaining Portugal in Monday’s draw against Serbia.
“He’s already started to move but he won’t return to training until he feels fit. There is a risk he’ll miss the first leg against Ajax,” Allegri said.
“We have to be careful because we’re in a tricky period of the season. It’s better he misses one match and comes back when he’s 100 per cent so there’s no risk of a repeat injury. It’s better to miss one match than two months,” Allegri added.
Serie A champions Juventus travel to Amsterdam before hosting the Dutch outfit on April 16 with the Italians looking for a first European title since 1996.
Juve has virtually wrapped up the Scudetto crown for an eighth straight season as they hold a 15-point lead over second place Napoli with 10 games of the season to go and face lowly Empoli on Saturday.