Ronaldo is yet to open his account for Juve in European competition, having been sent off against Valencia before helping his side past United at Old Trafford last month.
The 33-year-old was sent off in his first Champions League match for the Italian giants, a 2-0 win over Valencia, and was suspended for the club's second match of the campaign.
Juventus managed to win both of those matches without him, and the Portuguese superstar made his return to Europe's top club competition in a 1-0 win over former club Manchester United at Old Trafford.
Ronaldo's cross from the right side set the table for Paulo Dybala's winner in that contest, but he extended his personal scoreless run the Champions League to five matches, his last goal coming against his current club back in April.
Allegri, however, doesn't believe his star man's run without a goal will continue for much longer and has backed him to find the net against Jose Mourinho's outfit on Wednesday.
"He will score [on Wednesday]," he said.
"He didn't play the first match, he didn't play the second, he played only one match. And he provided an assist.
"I mean, it's not like he played many Champions League matches so far."
Manchester United have won their last two Champions League matches away to Juventus, in 1999 and 2003, but the Serie A side have lost just once in 35 home games in the competition since last facing United in Turin.
But Jose Mourinho's side enter the contest on a barren run in Europe, having failed to score in each of their last two matches in the competition.
Juventus hold a perfect record in Group H and will seal their place in the knockout stage if they avoid defeat against United.
Should Ronaldo score in Wednesday's contest he will extend his own record as the top scorer in Champions League history.
To date he has netted 120 times, with Barcelona star Lionel Messi sitting just behind him on the all-time list .