The Bianconeri announced Ronaldo as part of their squad for the first leg at the Johan Cruyff Arena in Amsterdam after he missed their last five Serie A games because of a hamstring injury.
Allegri said on Tuesday his star would be fit to face Ajax from the first whistle:
The 34-year-old pulled Juve back from a 2-0 deficit in their round-of-16 meeting with Atletico Madrid, netting a hat-trick in their 3-0 victory in the reverse fixture to see them qualify.
Ronaldo was injured while on international duty with Portugal in March but was hoping to come back and impact on his quest to clinch a fourth successive Champions League title.
He scored just one goal in his first six European appearances for the Bianconeri, but the veteran exploded onto the Champions League stage after his triple against old rivals Atleti forced Juve into the last eight.
Ajax manager Erik ten Hag said in preparation for Wednesday's duel that he was hoping Ronaldo was fit so they could face the best challenge their opponent could muster, via Goal:
Ronaldo's recent run out of Allegri's lineup is his first injury-enforced absence since arriving in Turin. The only other fixture he missed was against Young Boys in October after he was sent off against Valencia in his European debut for Juve.
The forward leads the Champions League's all-time top scorer list on 124 goals, 105 of which came at Real.
He was a driving force in Real's three consecutive Champions League titles under Zinedine Zidane and is looking to maintain status as one of Europe's most influential figures, as shown by BT Sport (UK only):
Ajax were forced to come back from behind in Madrid after they lost their opening leg against Los Blancos 2-1. Juve's own second-leg comeback against Atletico was evidence the Dutch giants can ill afford to make the same slow start in the last eight.
Juve defender Daniele Rugani was pleased to have his side's talisman back in the XI:
The Bianconeri won all but one of Ronaldo's five missed matches in Serie A and are now just one point from securing an eighth consecutive Scudetti.
Ronaldo's time away away from the team could lead to some rust in Amsterdam, with the competition's all-time top scorer back in the frame to add to his record tally.