Ronaldo sustained a thigh injury just half an hour into Portugal's 1-1 draw with Serbia during the international break but initially looked as though he would recover in time to face the Eredivisie outfit.
But Bianconeri boss Allegri gave a comprehensive update on the fitness of his squad in the build-up to his side's Serie A clash with Cagliari on Tuesday, with their Champions League encounter firmly in mind.
"Cristiano is working to get back, we hope to have him ready for Ajax, but it will be difficult," Allegri said.
"Today's exam is much better, the leg is much better, then there is a clinical examination and the player's symptoms.
"There are still nine days to go, so we still have time to evaluate things together. He is doing things he can do right now, it is normal that you have to be careful in everything and evaluate pros and cons.
"Douglas Costa is in good condition and should be available for that game," he added. "Sami Khedira trained yesterday and could be back for AC Milan [on April 6].
"[Juan] Cuadrado is working well and it's only a matter of time until he's back with the group. I'll see how Paulo Dybala is today, whilst Mario Mandzukic had a bit of a fever so will need to be evaluated."
Former Milan bossAllegri may well have a near-full complement if things progress as positively as his comments suggested for the majority of the fitness doubts in the squad, but the fact that his five-time Ballon d'Or winner seems the least likely to recover in time was an obvious concern.
Despite a slow start to the European season with just one goal in the group stages, Ronaldo proved his Champions League mettle against Atletico Madrid in the last 16 with match-winning hat-trick in Turin to turn the tie in Juve's favour.
And Allegri would be hopeful of him being available for the first leg in Amsterdam, particularly given Ajax's shock win over Real Madrid in the previous round.