Ronaldo suffered a thigh injury playing for Portugal late last month and has not featured for Juventus since netting a stunning hat-trick to dump out Atletico Madrid in the last 16 in early March, missing his club's last four Serie A matches.
"Cristiano has trained with the team and is available. As long as nothing happens tonight or tomorrow morning, he will start," Juventus coach Massimiliano Allegri said on Tuesday evening at the Johan Cruyff Arena.
The return of the competition's all-time top scorer is a big boost for Juve, who without him have moved to within a point of an eighth straight Serie A title.
"Cristiano looks the same as ever. He's in good shape, a very important player for us, as everyone knows, and we are happy he'll be with us," said defender Daniele Rugani, who is set to start in place of the injured Giorgio Chiellini.
Ronaldo has scored 24 goals for Juve this season and 124 in total in the Champions League, a tournament he has won five times, including in the last three seasons with Real.
His goals in the last 16 rescued Juventus after they lost 2-0 at Atletico in the first leg, and Allegri warned his team not to repeat that performance here. "We cannot think of leaving here without a goal," he said.
If Ronaldo's move to Juventus for 100 million euros ($117 million) heralded the end of an era in Madrid, the Spanish side's reign as kings of Europe was really finished when they were humbled by Ajax in the last round.
The Dutch club recorded a stunning 4-1 win away in the second leg to advance 5-3 on aggregate and reach their first Champions League quarter-final since 2003.
- Rich history -
Ajax have won the trophy four times, beating Juventus 1-0 in the 1973 final in Belgrade with Cruyff as captain and Johnny Rep scoring the winner.
However, it is the 1996 final defeat on penalties against Juve in Rome which really still rankles. Ajax were the holders at the time but felt they were cheated following allegations of doping by their opponents, although Juve always denied those.
It was only just after that final that Ajax moved to their current home, but it is a stadium dripping with history, with the bust of Cruyff at the entrance and the photographs of past stars lining the walls.
Yet in the modern game Ajax are the gatecrashers, the first Dutch quarter-finalists for 12 years.
"For Ajax it is very important to play the quarter-finals but also financially the circumstances have really changed in recent years and for a country like the Netherlands it's really difficult to get to this stage," coach Erik ten Hag said.
- 'Football has to surprise people' -
Ajax's revenue for last season was under 100 million euros, barely a quarter of Juve's.
"It's not only good for Ajax but also for the Champions League as it's not good that each year the same seven or eight clubs get to the quarter-finals and win the Champions League," insisted the coach.
"That is not what supporters want. Football has to surprise people and it's very nice that we can be the surprise this year."
Ajax's success this season has come thanks to a mixture of young stars like captain Matthijs de Ligt and Barcelona-bound Frenkie de Jong, and more experienced players like Dusan Tadic.
They are still fighting on three fronts, and in some ways this tie is a bonus for a club whose priority is really winning a first Dutch title since 2014.
"We have a lot of young players and we were lacking the middle category in terms of age. This season you can see the success of that balance," said Ten Hag of his squad.