Ronaldo was sent off in Juve's 2-0 group stage win over Valencia on matchday one after an innocuous clash with Jeison Murillo.
The 33-year-old attacker was visibly shocked by the red, which could have resulted in a multiple-match ban had UEFA's disciplinary committee opted to extend it.
However, it was confirmed on Thursday that Ronaldo will only be suspended for the visit of Young Boys on Tuesday, meaning he will be able to face former employers United both home and away.
"I'm happy, it's right that he gets to play against Manchester United," Allegri told reporters in his pre-match news conference ahead of facing Napoli on Saturday.
"He's doing well, even in [the game against] Bologna he had a good game. It's right that he plays for the team, just as the team plays for him.
"Then we have to improve in some things, we have to serve him better, but we are together recently, we have all the time to improve."
Saturday's clash sees Allegri come up against Carlo Ancelotti for the first time since the latter's return to Italy this summer.
Ancelotti was seen as a coup for Serie A, much like Ronaldo, but Allegri does not believe one of them is more important to Italian football than the other.
"Ancelotti's return was good for all Italian football, Ronaldo has brought attention to our football, also attracting other champions," Allegri said.
"I don't know if it [Serie A] is the most beautiful championship, but it is certainly the most difficult to play. There is great attention from all the coaches, you have to work hard to win the games."
Since Ronaldo's signing, Allegri has often had to leave out big-name players in order to accommodate the Portugal attacker, with Paulo Dybala suffering the most.
But, while the coach does not find it painful leaving players out, he is not ruling out potentially playing with Ronaldo, Dybala, Mario Mandzukic and Federico Bernardeschi all at the same time.
"Painful? No, we have many solutions," he added. "In the last few games Bernardeschi is out, and he is the fittest of all, then we have outside important players like [Sami] Khedira and Douglas Costa.
"I still have to make choices. It's possible [all the attackers play]. The team is finding more balance."